Molecular and Cellular Transitions from ES Cells to Mature Functioning Human Neurons
Human embryonic stem cells (hESCs) are pluripotent entities, capable of generating a whole-body spectrum of distinct cell types. We have developmental procedures for inducing hESCs to develop into pure populations of human neural stem cells (hNS), a step required for generating authentic mature human neurons. Several protocols have currently been developed to differentiate hESCs to what appear to be differentiated dopaminergic neurons (important in Parkinson’s disease (PD) and cholinergic motor neurons (important in Amyolateral Sclerosis (ALS) in culture dishes. We have developed methods to stably insert new genes in hESC and we have demonstrated that these transgenic cells can become mature neurons in culture dishes. We plan to over express alpha synuclein and other genes associated with PD and superoxide dismutase (a gene mutated in ALS) into hESCs and then differentiate these cells to neurons, and more specifically to dopaminergic neurons and cholinergic neurons using existing protocols. These transgenic cells can be used not only for the discovery of cellular and molecular causes for dopaminergic or cholinergic cell damage and death in these devastating diseases, but also can be used as assays to screen chemical libraries to find novels drugs that may protect against the degenerative process. Until recently the investigation of the differentiation of these human cells has only been observed in culture dishes or during tumor formation. Our recent results show that hESC implanted in the brains of mice can survive and become active functional human neurons that successfully integrate into the adult mouse forebrain. This method of transplantation to generate models of human disease will permit the study of human neural development in a living environment, paving the way for the generation of new models of human neurodegenerative and psychiatric diseases. It also has the potential to speed up the screening process for therapeutic drugs.
We plan to develop procedures to induce human ES cells into mature functioning neurons that carry genes that cause the debilitating human neurological diseases, Parkinson’s disease and Amyolateral Sclerosis (ALS). We will use the cells to reveal the genes and molecular pathways inside the cells that are responsible for how the mutant genes cause damage to specific types of brain cells. We also will make the cells available to other researchers as well as biotech companies so that other investigators can use these cells to screen small molecule and chemical libraries to discover new drugs that can interfere with the pathology caused by these mutant cells that mimic human disease, in hopes of accelerating the pace of discovery.
Our research is focused on studying two debilitating diseases of the nervous system: Parkinson’s disease (PD) and Amyotrophic Lateral Sclerosis (ALS) also known as Lou Gehrig’s disease. While the causes and symptoms of these two conditions are very different, they share one aspect in common: patients gradually lose specific types of nerve cells, namely the so-called dopaminergic neurons in PD, and motor neurons in ALS. If we can find ways to protect the neurons from dying, we might be able to slow or even halt disease progression in ALS and PD patients. In the past two years, our lab has developed robust procedures to generate these two classes of neurons from human embryonic stem cells and we have been studying the molecular changes that govern their specialization. Since last year, we have been using neurons to elucidate the molecular mechanisms that underlie the demise of these cells.
ALS is one of the most common neuromuscular diseases, afflicting more than 30,000 Americans. Patients rapidly lose their motor neurons – the nerve cells that extend from the brain through the spinal cord to the muscles, thereby controlling their movement. Therapy options are extremely limited and people with ALS usually succumb to respiratory failure or pneumonia within three to five years from the onset of symptoms. Most ALS patients have no family history of ALS and carry no known genetic defects that may help explain why they develop the disease. However, a small number of ALS patients have mutations in the superoxide dismutase 1 (SOD1) gene, which encodes an enzyme that scavenges so-called free radicals – aggressive oxidizing molecules that are by-products of the cells’ normal metabolism. Researchers therefore believe that accumulation of these free radicals may damage motor neurons in ALS and contribute to their death.
To test this idea, we introduced the mutated form of the SOD1 gene into astrocytes – cells that provide metabolic and structural support to neurons – and cultured our stem cell-derived motor neurons along with these SOD1-mutant astrocytes. Indeed, while motor neurons grown on ‘normal’ astrocytes were fully viable, we saw widespread death of motor neurons in cocultures with ‘mutant’ astrocytes, along with elevated levels of free radicals. We think that this is due to our mutant astrocytes being causing inflammation, and so our future efforts are focused on understanding the role of the immune system, specifically the function of microglia – the resident immune cells of the brain and spinal cord – in our co-cultures with human motor neurons. We are very excited about these results because they show that our cocultures may be a very useful tool to screen drugs that may counteract the neurotoxicity caused by inflammation and free radicals. We have already begun testing several known antioxidants, and found some of them to be very effective in improving motor neuron survival in the culture dish. Such compounds may ultimately improve the condition of ALS patients.
PD is the second most common neurodegenerative disease and develops when neurons in the brain, and in particular, in a part of the brain known as the substantia nigra die. These neurons are called dopaminergic because they produce dopamine, a molecule that is necessary for coordinated body movement. Many dopaminergic neurons are already lost when patients develop PD symptoms, which include trembling, stiffness, and slow movement. Around one million Americans are currently suffering from PD, and 60,000 new cases are diagnosed each year. While several surgical and pharmacological treatment options exist, they cannot slow or halt disease progression and are instead aimed at treating the symptoms. The exact causes for neuron death in PD are unknown but among others inflammation in the affected brain area may play a role in disease progression.
In a joint effort with the laboratories of Christopher Glass and Michael Rosenfeld at the University of California, San Diego, we showed using animal experiments that a protein called Nurr1 is crucial for the development and survival of dopaminergic neurons. We found that the Nurr1 gene is turned on by inflammatory signals and suppresses genes that encode neurotoxic factors. Microglia are the major initiators of the neurotoxic response to inflammatory stimuli, which is then amplified by astrocytes. Thus our findings reveal an important role for Nurr1 in microglia and astrocytes to protect dopaminergic neurons from exaggerated production of inflammation-induced neurotoxic mediators. We are now using human embryonic stem cell-derived dopaminergic neurons, cultured along with human atrocytes and microglia to test whether we can demonstrate this positive role of Nurr1 in a culture dish as well.
We are investigating the molecular mechanisms underlying two major neurological diseases: Parkinson’s disease (PD) and Amyotrophic Lateral Sclerosis (ALS), also known as Lou Gehrig’s disease. In the past year, we have taken our previously developed human embryonic stem cell (hESC)-based cell culture model for PD and ALS another step further: we have begun building an assay system that may eventually allow both the identification of biomarkers for early diagnosis and the screening of drug candidates for ALS and PD. By transplanting hESC-derived neurons into live animals and brain slices, we have also made first inroads into recapitulating the disease processes in animal model systems.
While the causes and symptoms of ALS and PD are very different, they share one aspect in common: in both, patients gradually lose specific types of nerve cells, namely, the so-called dopaminergic neurons in PD, and motor neurons in ALS; it this neuron death that causes both diseases. Previously, we showed with our hESC-based cell culture system that an inflammatory response in astrocytes (the brain cells that provide metabolic and structural support to neurons) is involved in loss of motor neurons. Similarly, we demonstrated that microglia (the brain’s immune cells) and astrocytes together protect dopaminergic neurons from exaggerated production of inflammation-induced neurotoxic mediators. This function of astrocytes and microglia was dependent on a protein called Nurr1: we found that the Nurr1 gene is turned on by inflammatory signals and suppresses genes that encode neurotoxic factors.
We have now begun to characterize in depth the specific signaling molecules that communicate the inflammation cue from the glial cells to neurons. To do this, we cultured astrocytes and microglia in the petri dish, induced inflammation and collected cell culture supernatants from the ‘inflamed’ and normal cells. We then measured the levels of specific so-called cytokines, the inflammatory signaling molecules secreted by the glial cells. Once we have obtained a characteristic cytokine ‘signature’ of disease-associated glial cells, we can begin to unravel the molecular pathways that lead to inflammation. Thus our research may lead to the discovery of early diagnostic markers and enable drug screening for compounds that suppress or prevent these neurotoxic inflammatory processes.
Our cell culture assays have provided a great deal of insight into the signaling cascades that eventually lead to neuron death. However, they probably cannot fully recapitulate the complex interplay between the neurons and the cellular environment in which they reside within the brain. We have therefore begun to transplant hESC-derived neurons into the brains of mice. Our results indicate that the neurons rapidly extended processes and developed dendritic branches and axons that integrated into the existing neuronal network. In the coming year, we plan to build on these results, using our hESC-derived neuronal models of PD and ALS to better understand mechanisms of dysregulation. Specifically, we will examine alterations in synapse formation, cell survival, and neuron maturation. We will also devise strategies for functional recovery and rescue in the context of the living animal.
We are investigating the molecular mechanisms underlying two major neurological diseases: Parkinson’s disease (PD) and Amyotrophic Lateral Sclerosis (ALS), also known as Lou Gehrig’s disease. In the past year, we took our human embryonic stem cell (hESC)-based neural cell culture model for PD and ALS another step further and built sensitive and quantitative assays that can allow for the screening of drug candidates for ALS and PD. We have also consistently improved our transplanting techniques and are now able to detect functional, electrophysiologically active, hESC-derived neurons in live animals. This experiment was crucial to show that, under our culture conditions, human neurons derived from embryonic stem cells were able to integrate and form meaningful connections with other neurons in a given adult brain environment.
Moreover, we are now performing an in-depth characterization of the specific signaling molecules that communicate the inflammation cues from the glial cells to neurons in the presence of ALS-causing mutations (SOD1G37R) and PD-causing mutations (recombinant alfa-synuclein). In this report we have explored another functional assay to measure glial function and inflammatory response using astrocytes that express ALS-causing mutations. In addition, we report here that adding PD-causing mutagens to mixed cultures of human neurons and astrocytes results in the death of dopaminergic neurons, the type of neurons affected in PD. We are currently testing new compounds that can decrease the neuronal toxicity observed.
Our research may not only lead to the discovery of early diagnostic markers but also enable drug screening for compounds that suppress or prevent these neurotoxic inflammatory processes.
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