Immunology of neural stem cell fate and function

The adult brain and spinal cord do not regenerate after major injury or disease but there are stem cells present in the brain that are capable of generating new brain tissues. In some regions of the brain, these brain-specific stem cells are continuously producing new nerve cells or “neurons”, the type of cell that forms the brain’s internal circuitry. The process of generating new neurons is called neurogenesis and our goal in this grant is to learn how neurogenesis is naturally maintained and controlled. Using the information we gain, our ultimate goal is to stimulate neurogenesis and repair in other areas of the brain, either by activating resident stem cells or by improving the effectiveness of stem cell transplants. One region of the brain where neurogenesis naturally occurs is called the hippocampus. This small brain structure is important for short term learning and memory. It is one of the brain regions affected in Alzheimer’s disease. Our laboratory was one of the first groups to show that brain tissue inflammation that accompanies injury or disease strongly inhibits neurogenesis in this area. We have gone on to show that the production of neurons by stem cell transplants is also strongly inhibited by tissue inflammation. Tissue inflammation is caused by the natural activation of the immune system when cells or tissues are damaged. One focus of our research in this grant is to identify factors produced by the immune system that inhibit neurogenesis and to determine if blocking these factors will enhance endogenous neurogenesis or neurogenesis from stem cell transplants. The concepts are summarized in a recent publication from our group (Carpentier and Palmer, Neuron 2009) We are also using genetically mutant mice to determine if the absence of specific inflammatory molecules can promotes neurogenesis and have found that the absence of one immune molecule does indeed improve neurogenesis. There are experimental drugs that block the activity of this molecule and we hope to test these drugs in the coming year for their ability to promote neurogenesis. A second focus of this project is to identify signals that promote neurogenesis. The stem cells in the hippocampus provide several intriguing leads. Our research has uncovered two important neurogenesis-promoting signals present in the hippocampus. The first is a specialized matrix of proteins that are produced in the hippocampus. This matrix acts like a “gate keeper” to define where stem cells should (or should not) generate new neurons. It is also known that stem cells in a petri dish will generate more neurons if one treats them with drugs that mimic brain circuit activity. However, no one had directly confirmed this in animals. To test this, we have produced a genetically modified mouse in which we can control activity in the hippocampal neural circuitry (Haditsch et al Molecular and Cellular Neuroscience 2009). Using this mouse, our ongoing research has identified proteins released from active neurons that promote neurogenesis. Again, we have found experimental drugs used for other purposes that appear to mimic the neurogenesis-promoting aspects of circuit activity and our hope is that these can be used to promote neurogenesis for therapy. Of course, the ultimate goal of our work is to translate our findings into treatments for brain injury or disease and the final aim of our project is use these combined observations to determine if stem cell transplants can be effectively harnessed for repair. The first step for transplantation strategies is to determine if stem cell derived transplants can work to produce new circuitry. We have just completed studies showing that the methods used to prepare embryonic stem cells for transplant have a large effect on the ability of the transplanted cells to form neurons that make correct connections within the brain. Stem cells must be programmed correctly before transplant if they are to function as desired. We are also performing experiments to understand the immunology of stem cell transplantation itself. At present, stem cells being used in clinical trials come from either human embryonic stem cells isolated from a human blastocyst or from neural stem cells isolated from human fetal brain. Neither of these sources would be perfectly matched to the recipient but recent discoveries have shown that stem cells can be generated from the skin fibroblasts of a patient. These “induced pluripotent stem cells” or iPSC are perfectly matched but the technology and clinical applications are lagging far behind. Even as fetal or embryonic stem cells are entering clinical trials, the impact of transplant matching and immunology remains an unanswered question and our ongoing work may uncover methods that can improve transplant outcome even if the cells are poorly matched to the recipient.

The adult brain and spinal cord do not regenerate after major injury or disease but there are stem cells present in the brain that are capable of repairing damaged neural networks. One region of the brain where new neurons are continuously produced is called the hippocampus. Unique signals within the hippocampus promote neurogenesis (i.e., the production of new nerve cells) and our hope is to identify and use these signals to promote neurogenesis following injury or disease. In addition, our laboratory has also shown that the tissue inflammation that accompanies injury or disease strongly inhibits neurogenesis and our ongoing research has two goals: First, to identify factors that naturally promote neurogenesis and apply these factors to enhance natural repair and/or improve stem cell transplant outcome. Second, we hope to identify inhibitory factors produced by the immune system during tissue inflammation and develop better interventions to block these signals and promote neural circuit repair when an injury or disease process is present. Activities in year 3 have focused primarily on identifying immune mechanisms that impair neurogenesis when stem cell transplants are not perfectly matched to the recipient. Under normal circumstances, tissue transplants are rejected by the immune system if they do not closely match the recipient. This limits the clinical use of donor organs such as heart, liver, lung. However, transplants of cells to the brain are not rejected, even if they are poorly matched. However, we have found that immune signaling is not absent in the brain but simply different than in other areas of the body. While graft rejection does not occur, neurogenesis is strongly inhibited. We have found that there are two immune processes that are relevant to neurogenesis: 1) immune activation/inflammation that stimulates the production of immune signaling molecules impairs neurogenesis and 2) the elimination of transplanted cells that are “not exactly like me” by natural killer cells may selectively eliminate the newly forming neurons. In year 3 we have explored the interactions of NPCs with a variety of immune cell types, including T cells and NK cells. We have found that NPCs are recognized by both cell types and are now exploring methods to attenuate the effects of immune cell response following stem cell transplantation in cell transplant models. These include studies to restore neural stem cell function following therapy for brain cancer, studies to enhance regeneration and repair following spinal injury, and studies to improve stem cell therapy for Parkinson’s disease.

Research supported by this CIRM grant focused on understanding the immune signaling that influences neural stem cells in the brain and spinal cord. Stem cells are important for the natural replacement of brain cells that may be lost due to age, disease, or injury. Stem cells may also be useful in repairing the brain. Our research has shown that immune signaling and the tissue inflammation that accompanies injury or disease strongly inhibits “neurogenesis” or the production of new neurons from stem cells. Our goals in this work are to identify factors that naturally promote neurogenesis and apply these factors to enhance natural repair and/or improve the utility of stem cell transplants for therapy. One of our strategies is to identify inhibitory factors produced by the immune system during tissue inflammation and develop better interventions to block these signals and promote neural circuit repair when an injury or disease process is present. As a result of the research supported by CIRM we have identified three novel uses for drugs that have already been used in clinical settings. These drugs increase the number of new neurons produced by stem cells in the brain by either blocking negative immune signals or by directly stimulating neurogenesis. Experiments started in the final year of this grant will now continue in studies of stem cell transplants that are not perfectly matched to the recipient. Many cell transplant therapies for neurological disease or injury have utilized cells from another individual rather than a patient’s own cells. Tissue transplants are normally rejected by the immune system if they do not closely match the recipient and we have found that classical methods to protect grafted cells may not work as well as anticipated. Using the drugs identified in our CIRM funded research, we will next test whether controlling immune system effects can further improve the efficacy of stem cell therapies that have been engineered to treat stroke, spinal injury or Parkinson’s disease.

A central goal in stem cell therapy for neurological disease or injury is to understand how to optimize the survival and functional integration of stem cell-derived neurons. Neurons are the nervous system cells that form the interconnected networks that control cognition and behavior. Neurons are also the most sensitive cell type in the brain and the loss of function that accompanies disease or injury is due, in large part, to the inability of the brain to replace neurons that die. We have found that the inflammation that accompanies tissue damage selectively impairs the survival of newly generated neurons and research supported by this CIRM grant focused on understanding why immune signaling has such a detrimental effect in the brain and spinal cord. The process of generating new neurons is termed neurogenesis. Neurogenesis is mediated by tissue-specific stem cells or “neural stem cells”. Neural stem cells mediate the formation of the brain and spinal cord during development. Neural stem cells are also important for the natural replacement of many types of brain cells that may be lost due to age, disease, or injury. Neural stem cells also continuously generate new neurons in select regions of the adult brain and our research focus has been to understand this natural process in order to improve the survival and functional benefit of new neurons that are transplanted to the damaged nervous system. Research supported under this CIRM Comprehensive Grant has shown that immune signaling and the tissue inflammation that accompanies injury or disease strongly inhibits neurogenesis and our goals in this work are to identify factors that naturally promote neurogenesis and apply these factors to enhance natural repair and/or improve the utility of stem cell transplants for therapy. One of our strategies is to identify inhibitory factors produced by the immune system during tissue inflammation and develop better interventions to block these signals and promote neural circuit repair when an injury or disease process is present. Our earlier studies have led to several specific predictions about the cells and/or molecular signals that inhibit young neuron survival in the damaged brain. Experiments that that continued into a 6 month extension of this research tested whether genetically eliminating the implicated immune cell type or signaling molecule improves transplant outcome. Several drugs were also identified for their ability to protect newly generated neurons and enhance their survival and integration. Using genetic tools, we have confirmed that a specific type of immune cell selectively targets young transplanted neurons. Future studies will test drugs that selectively target this cell type with hopes of improving stem cell-based therapies. We have also confirmed that two clinically approved drugs which modulate tissue inflammation are beneficial in two transplant models. The first involves a model of childhood brain injury caused by cancer treatments. The young brain is particularly sensitive radiation and chemotherapy. Young cancer survivors often have permanent neurological problems. Stem cell grafts may be useful to replace neural stem cells that have been killed by the cancer treatment. We are also testing transplants of stem cell-derived neurons in a model of Parkinson’s disease. In both model systems, the drug-based interventions protected the transplanted cells and future studies will test these interventions in several additional stem cell therapy models, including stroke and spinal cord injury.