Circuitry and Brain-Body Interactions

Study Rationale: The motor symptoms of Parkinson’s disease (PD) result from the degeneration of the dopamine-producing neurons in a brain area called the substantia nigra pars compacta (SNc). Recent findings suggest that the SNc is diverse and is comprised of dopamine neurons with distinct properties. How these dopamine neuron “subtypes” contribute to movement, how they are affected in PD, and how they are modified by deep brain stimulation (DBS) remains unknown.

Hypothesis: We will determine whether (a) the SNc is comprised of pro-motor and anti-motor dopamine neuron subtypes and (b) selective loss of pro-motor neurons in PD causes an imbalance in dopamine neuron subtypes that underlies the motor symptoms of PD.

Study Design: We will separate these neurons into their distinct genetic subtypes, which will allow us to study their specific physiological, anatomical, and functional properties. We will also determine the molecular and circuit mechanisms underlying the dysfunction of dopamine neurons in a mouse model of PD (LRRK2 model). Additionally, we will explore whether deep brain stimulation of dopamine neuron inputs contributes to the therapeutic efficacy of this treatment.

Impact on Diagnosis: First, our work will identify which dopamine neuron subtypes degenerate and which circuits are dysregulated in PD. This knowledge will be important for understanding the pattern of SNc neuron loss in PD and efficacy of DBS in patients. By using a LRRK2 model, our studies also will identify the molecular targets of the hyperactive LRRK2 enzyme, which will be critical for the optimization of LRRK2 inhibitor drugs and their application in patients.

Study Rationale: Many people with Parkinson’s disease (PD) develop untreatable cognitive symptoms, including problems with attention, decision-making, and dementia at late stages of the disease, due to changes in a crucial part of the brain, the cerebral cortex. Evidence suggests that aggregation of the protein alpha-synuclein in vulnerable nerve cells interferes with their health and damages the cellular networks required for normal brain function. However, the relationships between alpha-synuclein, vulnerable cells, and network activity are not understood. By identifying and understanding the causes and effects of this damage in brain networks, and affected nerve cells and their connections, we aim to enable therapies that directly target this disorder.

Hypothesis: We hypothesize that networks of nerve cells in the cortex of the brain become dysfunctional because of damage caused by pathological deposits of the protein alpha-synuclein in vulnerable cells.

Study Design: We will assess how alpha-synuclein pathology progressively impairs network function in the brain cortex and identify the features distinguishing vulnerable from resilient cells using innovative technologies, including imaging of activity in the live brain, measurements of attention, profiling of different cell types and their contents, and high-resolution microscopy of neuronal connections. We will integrate these data using advanced computational methods to design and test cell-specific interventions to restore the function of the disrupted networks. This study will reveal mechanisms of cortical network damage in Parkinson’s disease and will identify the types of nerve cells suited for therapeutic intervention.

Impact on Diagnosis: Diagnostic biomarkers for PD can leverage our findings of which types of neurons, their connections, and molecular markers are most affected. For treatment, our work will define novel mechanisms that can directly restore network function by targeting specific types of cortical nerve cells and their connections.

Study Rationale: While neurodegeneration in Parkinson’s disease (PD) affects many cells, dopamine neurons are particularly vulnerable, and their loss drives many of the major motor difficulties in PD. To date, the inner workings of the dopamine neurons themselves have been extensively studied to identify sources for this selective vulnerability. However, neurons in the brain are heavily interconnected and interdependent with their surrounding cells and circuitry. Understanding how the “neighborhood” in which dopamine neurons live and function influences their well-being is a critical missing piece in the puzzle of PD.

Hypothesis: We hypothesize that circuit properties of incoming neuronal connections (synapses), surrounding non-neuronal cells (glial cells), and key modulatory cells (those that produce the chemical signal, acetylcholine) contribute to dopamine neuron loss in PD.

Study Design: We will evaluate circuit contributions to dopamine neuron dysfunction in PD using state-of-the-art mouse genetic models and patient-derived stem cell models (organoids). Our team members bring unique, specialized expertise that allows us to isolate and manipulate each of these three components (synapses, glia, and neuromodulators) individually to test its role in dopamine neuron degeneration. In addition to functional manipulations, we will capture the molecular signatures of the connections dopamine neurons make with each of its “neighbors” to determine which are most disrupted in PD.

Impact on Diagnosis: Recognizing new processes that cause dopamine neuron demise in PD creates new opportunities for intervention. Our bidirectional tests of function may identify not only circuit properties that accelerate disease, but also identify factors that promote resistance to cell death.

Study Rationale: Nerve cells that produce the brain chemical dopamine die in people with Parkinson’s disease (PD). These nerve cells extend long and thin fibers called axons that release dopamine from thousands of different points, sending signals to other nerve cells in a brain area called the striatum. Many different types of cells and molecules in the striatum can directly control how dopamine is released, but we haven’t yet discovered which ones are the most important and how they are affected in Parkinson’s. By better understanding this cooperation between striatum and the release of dopamine from axons, we could provide new knowledge toward ways to restore normal function.

Hypothesis: We think that other molecules in the striatum play a very important role in controlling the release of dopamine, particularly for the types of dopamine axons that are most vulnerable in PD. We believe that this role is disrupted in the disease and could be targeted to rescue symptoms.

Study Design: Our international team will combine cutting-edge research methods in mice and human cells that allow us to study the biology behind Parkinson’s. We will measure dopamine and other signaling molecules in different areas of the striatum and work out what they do. This work will reveal the biological differences between vulnerable and resistant areas. We will use this knowledge to study the most promising molecules in mice that develop PD and in cells from people with PD, to then suggest new ways that might fix the problems with dopamine in the disease.

Impact on Diagnosis: Our discoveries will provide knowledge that may help to find new ways of treating Parkinson’s using medicines that target the key signaling molecules in striatum that control dopamine release.

Study Rationale: Previous work has shown how the loss of dopamine neurons affects brain activity. In this program, we will determine how brain activity influences the neurodegeneration that causes Parkinson’s disease (PD). To understand the onset of disease, we will identify the earliest changes in brain activity and use them to infer the mechanisms involved. We will also manipulate activity directly and determine how it interacts with known genes to produce degeneration.

Hypothesis: We hypothesize that abnormalities in neural activity do not simply reflect PD but actually cause the disease. Our lack of knowledge about the role of neural activity in Parkinson’s makes it difficult to understand how other, identified factors contribute to disease.

Study Design: We will use two models of PD, one based on over-expression of the pathogenic protein alpha-synuclein and the other based on a direct increase in activity. The strategy is to (a) identify the earliest events along the pathway to degeneration and to (b) correlate these with the selective vulnerability of particular neurons to PD. These approaches will reveal the processes specifically affected by PD. We will also determine how neural activity intersects with factors previously implicated in PD, providing a foundation to understand how they cause degeneration.

Impact on Diagnosis: With greater understanding around the onset of disease, we can further investigate how genetic and environmental factors conspire to produce PD. This will open entirely new areas to arrest and prevent the underlying degeneration.

Study Rationale: We recently discovered that it is possible to generate new neurons to rebuild the damaged neural circuitries in a Parkinson’s disease model. This establishes a new foundation for developing strategies to reverse the disease. Our proposed project will investigate how to change a cell’s identity to encourage it to become a neuron, how to make the right new type of neurons in the brain to rebuild different circuitries, and how to make new neurons that will not become sick again.

Hypothesis: We hypothesize that astrocytes, an abundant population of non-neuronal cells in the brain, store a latent program that allows them to become neurons if the right types of inducing signals are provided.

Study Design: We have designed five sets of experiments to address a series of fundamental questions on cell fate determination and reprogramming. We will analyze individual cells to elucidate key regulatory events responsible for those cells to become neurons. We will search for critical genes that make cell fate change less efficient so that we can improve the reprogramming efficiency by inhibiting the function of those genes. We will examine reprogramming in different brain regions to test their benefits to both motor and non-motor symptoms and develop strategies to make new and disease-resistant neurons.

Impact on Diagnosis: More information on cellular reprogramming will pave the way to rebuilding the neural circuitries lost to degeneration. If successful, this will lead to the development of a completely new cell replacement therapy against Parkinson’s disease.

Study Rationale: Although clinicians have long reported that Parkinson’s disease (PD) does not affect the brain alone, the field has only recently started to investigate gut dysfunction in experimental models of PD. Consequently, the anatomical and functional basis of gut-to-brain circuitry dysfunction in PD—such as network activity and dopamine signaling—remain poorly understood. Our team will characterize circuit mechanisms underlying gut-to-brain disease spread and progression in the earliest appearance of Parkinson’s symptoms.

Hypothesis: Our hypothesis is that environmental and genetic factors impact the connections between neurons in the enteric nervous system (ENS), which regulates the gastrointestinal tract. This disruption may increase susceptibility to PD triggers—including aggregation of alpha-synuclein, which is toxic to cells and can trigger PD symptoms and gut inflammation. This inflammation could, in turn, augment the toxic form of alpha-synuclein that circulates to the brain and causes dysfunctions in neural circuits and motor deficits.

Study Design: We will determine PD-relevant gut and brain anatomic and physiologic profiles in rodents, primates, and human cells by clarifying the anatomic pathways underlying gut-to-brain propagation of aggregated alpha-synuclein in mice, and by constructing anatomic and functional maps of macaque ENS and central nervous system PD-relevant circuits at single-cell resolution. We also will test whether disruption of ENS circuitry mitigates gut-brain disease outcome and evaluate whether spiny mice (a rodent model that can repair damaged tissue) show protection from PD-related gut-brain degeneration.

Impact on Diagnosis: Using powerful new technologies across multiple PD-relevant model systems, this project could uncover novel circuit mechanisms that mediate symptoms, and embolden new therapeutic options to slow, halt, or perhaps reverse peripheral symptoms of PD.

Study Rationale: A hallmark of Parkinson’s disease (PD) is the development of abnormal deposits throughout the brain composed of the alpha-synuclein protein. This pathology can be found in the gut too. Increasing evidence indicates that, in some patients, alpha-synuclein pathology may begin in the gut and spread to the brain through a nerve called the vagus, which directly connects the gut to the brain. An animal model developed by our team members reproduces many of these features seen in humans to allow study of these pathways and consequences of this advancing disease as it spreads throughout the brain.

Hypothesis: We hypothesize that specific vagus nerve cells are responsible for disease spread from gut to brain, with both vagus nerve activity and other factors such as gender and menopause affecting this spread, resulting in early symptoms such as sleep disorders seen in humans before development of movement problems.

Study Design: The first two goals will use genetic manipulation to study (a) what cells may be responsible for gut-to-brain spread of abnormal alpha-synuclein, (b) how disease spread affects normal vagus functions, and (c) how different levels of vagus activity influence disease spread. We will also study the consequences of this type of gut-brain spread on development of early symptoms that may occur before the movement problems, particularly sleep disorders. Given the reduced risk of PD in women prior to menopause, our final goal is to study these same problems in a novel animal model that mimics human menopause.

Impact on Diagnosis: The gene therapy methods used to block gut-brain spread in our studies could be applied non-invasively to patients with diagnosed presence of gut alpha-synuclein pathology to prevent disease spread. Our sleep and menopause studies will further identify opportunities for early intervention and possible hormonal approaches to limiting effects of disease spread.

Study Rationale: Emerging evidence supports the concept that alpha-synuclein pathology, a hallmark of Parkinson’s disease (PD), might originate in the gastrointestinal tract and spread to the brain. However, where and how pathological alpha-synuclein initially forms remains unclear. Certain gut microbiota and some environmental toxins have been associated with PD. We recently discovered that specialized sensory cells in the gut known as enteroendocrine cells (EECs), in direct contact with gut microbiota and different environmental toxins, might transfer misfolded alpha-synuclein to the nervous system in disease.

Hypothesis: We hypothesize that gut microbes and environmental toxins, alone or in combination, corrupt alpha-synuclein protein expressed in EECs to misfolded pathological forms that might spread to the nervous system in a novel circuit important in PD.

Study Design: Using a unique, large, and well-characterized collection of human data, we will identify potential triggers and alterations in gut microbiota and neuroinflammatory pathways that are associated with PD. We will test known and experimental triggers (toxins and microorganisms) in two complementary model systems (ex vivo cultures, and in vivo in mice) to determine effects on the formation and spread of pathological alpha-synuclein within a predictable circuit of interconnected cells vulnerable to disease.

Impact on Diagnosis: These studies will (a) establish how EECs are involved in the formation of pathological alpha-synuclein at the earliest stages of disease, (b) identify gut-associated toxins and microbes that contribute to PD, (c) develop novel “humanized” pre-clinical model systems, and (d) test two particularly promising experimental therapeutic approaches in the novel model systems.

Study Rationale: To slow the progression of Parkinson’s disease (PD), we need to know more about how it starts. It is well known now that many individuals with PD have had a significant reduction in their ability to smell years before their movement disorder starts. Here, our team will study the role of a PD-linked gene, alpha-synuclein, in odor processing, and how viruses in the nose may change how alpha-synuclein is handled within the odor-signaling cells of the brain in people with PD.

Hypothesis: We hypothesize that certain environmental triggers, including viruses, can start a chain reaction inside the nasal cavity by which a protein called alpha-synuclein begins to form insoluble clumps. This happens inside the nerve circuits that specifically process scent and then gradually spreads further inside the brain, thereby promoting Parkinson’s disease.

Study Design: We will first define the normal role of the alpha-synuclein protein in the scent-processing circuits of mice and humans. We will then answer whether so called Lewy bodies (i.e., alpha-synuclein clumps in nerve cells) contribute to the inability to smell. Third, we will study the areas of the human brain that are responsible for smell, including using MRI imaging. Lastly, we will perform studies with nasal fluids from people with PD and introduce these (and viruses) into mice, to see if these start a reaction that cause changes in the mouse olfactory system like what is seen in humans with PD, including loss of smell and changes in the alpha-synuclein protein. We will see if the inability to smell can progress to motor deficits.

Impact on Diagnosis: This study will delineate what causes the loss of sense of smell in PD. That will help us to develop new drugs to treat it and explore new ways to diagnose the disease, hopefully at a stage before the movement symptoms appear. We may also gain insights into risk factors for PD, which could lead to strategies to help people reduce their risk. We will also create new animal models that can be used by others.

Study Rationale: People with Parkinson’s disease have long been known to display remarkable motor abilities under special circumstances, such as smooth walking with certain visual or auditory cues. This phenomenon is called paradoxical kinesia. In addition, placebos can be surprisingly effective in treating the motor signs of the disease. We hypothesize that a specific neuroanatomical substrate supports paradoxical kinesia and the placebo effect. We plan to define this substrate and investigate its functional organization.

Hypothesis: We hypothesize that a specific neural circuit supports paradoxical kinesia and the placebo effect.

Study Design: We will use cutting-edge techniques to reveal the two distinct brain circuits that enable the basal ganglia to influence the control of voluntary movement in primates. Next, we will record the electrical and chemical activity of basal ganglia neurons in the best animal model of Parkinson’s disease. In addition, we will determine the molecular signatures of basal ganglia neurons that are affected by the disease and those that are left intact. Finally, we will image neural activity in human subjects affected by the disease to determine the full range of strategies that could be used to improve basal ganglia function.

Impact on Diagnosis: Our results could re-shape paradigms for therapeutic development and attempts to influence disease progression. Importantly, our results have the potential to use basal ganglia circuits that are untouched by the disease to promote recovery of more normal motor function.

Study Rationale: Parkinson’s disease (PD) begins decades before it compromises the ability to move about in the world and sleep through the night. Understanding how the dysfunction of brain circuits begins and then evolves to cause difficulty in moving and sleeping will allow us to diagnose PD earlier—increasing our chances of halting disease progression—and to better treat the disease once it appears.

Hypothesis: The progressive damage to dopamine-releasing neurons results in staged disruption of neural circuits in larger and larger parts of the brain, ultimately leading to both motor and sleep deficits characteristic of PD.

Study Design: Our plan is to study a new genetically engineered mouse model that manifests a progressive, levodopa-responsive parkinsonism. Importantly, this mouse faithfully reproduces the human staging of pathology in key brain circuits. Using the most advanced methods available for studying and manipulating genetically defined brain circuits, the causal linkage between circuit dysfunction and motor and sleep behavior will be determined.

Impact on Diagnosis: A better understanding of how the circuit dysfunction underlying PD is staged should allow earlier diagnosis—enhancing the potential benefit of disease-modifying therapies—and better treatment strategies for later-stage patients.

Study Rationale: Specific brain circuits that are highly melanized (build-up of the dark pigment neuromelanin) with age are primarily affected, particularly early, in Parkinson’s disease (PD). Models incorporating this aspect of PD have only been developed recently and show that increased neuromelanin production causes neurodegenerative changes consistent with Parkinson’s. The regulators of cellular neuromelanin metabolism have not been determined, the effect of neuromelanin on normal activity in these pathways has not been defined, the potential for neuromelanin aggregates to increase alpha-synuclein accumulation has not been evaluated, and the impact of extracellular neuromelanin on detrimental inflammatory processes has not been assessed.

Hypothesis: Activity in melanized brain circuits is a dominant factor in the initiation of PD and sustains its progression by seeding pathology in connected regions and providing the stimulus for chronic inflammation. Manipulating neuromelanin production and/or brain circuit activity can ameliorate these deficits.

Study Design: Parallel experiments will be performed in mice and non-human primates in which neuromelanin production has been induced for comparison with neuromelanin-producing neurons in people with prodromal and early Parkinson’s. To test whether activity in melanized brain circuits is a dominant factor in the initiation of PD, spatiotemporal activity mapping, imaging, and other techniques will be used, and manipulating neuromelanin production and/or brain circuit activity will be assessed as potential treatments. To determine if neurons spread PD pathology through their connectivity, seeding experiments will be performed and impacts on behavior and neurodegeneration assessed. To determine how non-neuronal mechanisms are involved in disease progression, high-resolution microscopy and cell-specific details of changes in extracellular spaces and infiltration of non-neuronal cells into the brain will be assessed.

Impact on Diagnosis: Diagnosis of neuromelanin changes in the brain are already being assessed for their diagnostic potential, but this study will determine their focus and rate of change with respect to neural activity and clinical features. We will also identify if reducing neuromelanin levels stabilizes pathology and restores brain activity.

Study Rationale: The outer mantle of the brain, the cerebral cortex, plays a significant role in selecting and controlling movements. Changes in the activity of cortical neurons are key to disorders of movement, especially Parkinson’s disease (PD). It is unknown, however, which specific cell types are involved and how their activity changes during the course of the disease. In these experiments, we will use new technologies to study large groups of specific types of cortical neurons (for example, those that send fibers to the spinal cord) and explore how their activity and morphology change in animal models of chronic PD.

Hypothesis: Our hypothesis is that groups of cortical neurons that send fibers to the spinal cord—unlike those that send projections to the striatum—start to show abnormal activity and undergo morphological changes in connections that provide inputs to them when parkinsonism develops.

Study Design: We will measure the anatomical and functional characteristics of neurons in the motor cortex in animal models of slowly progressive PD. Optical imaging methods as well as electrophysiologic recordings will allow us to measure the activity patterns of large groups of individual cortical neurons, while parallel anatomical studies will identify the reshaping of connections to different families of cortical neurons before and during the development of parkinsonism. Computational analysis will allow us to put the findings together in computer simulations that will help us to understand the cortical circuit abnormalities that contribute to PD.

Impact on Diagnosis: A better understanding of how movement problems in PD develop is key to developing more effective methods to control them. Characterizing the abnormalities in specific families of cortical neurons may allow us to develop new therapies that target the affected circuits through deep brain stimulation, pharmacologic, or genetic methods.