Mazen A. Kheirbek - US grants

[unreadable] DESCRIPTION (provided by applicant): My project aims to investigate the neurobiological bases that underlie the balance between plasticity and stability in memory formation and maintenance. In mammals, hippocampus-dependent memory is mainly declarative, which is easy to form and forget (i.e. highly plastic). In contrast, striatum-dependent memory is mainly procedural or motor habits which form incrementally and hard to extinguish (i.e. highly stable). I hypothesize that adenylyl cyclases (AC) play an important role in this balance. AC1 is stimulated by Ca/CaM, and has high expression in the hippocampus, but is absent in the striatum, where ACS is the primary isoform. ACS is inhibited by Ca2+ and protein kinase A, two integral components of synaptic plasticity. I hypothesize that due to expression of ACS, there is a constraint on cAMP production in the striatum, causing reduced synaptic plasticity, and thus making striatum dependent memories more stable. I propose to test this using a transgenic approach: expressing AC1 in the striatum. I will test these mice biochemically for players in synaptic plasticity, and behaviorally in tasks that dissect striatum dependent memory. In terms of relevance of this research to public health, understanding the basis for corticostriatal plasticity is crucial for developing therapies for motorhabits related disorders that include Tourette Syndrome, obsessive compulsive disorder and addiction. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Depression is a major cause of disability in the world, and developing novel therapeutic approaches for its treatment is of utmost importance. Recent studies have suggested that interventions that have beneficial effects on mood such as exercise and chronic antidepressant treatment increase hippocampal neurogenesis. In addition, hippocampal neurogenesis is required for some of the behavioral effects of chronic antidepressant treatment. Adult-born granule cells (GC) in the DG exhibit a heightened synaptic plasticity during a critical window of their development, an enhanced plasticity mediated by the NR2B subunit-containing NMDA receptors. Yet, it remains unknown what role this increased excitability in adult-born GCs plays in behavior. This proposal will test the hypothesis that blocking the ability for adult-born GCs to contribute to plasticity in the DG has detrimental effects on cognitive function and antidepressant efficacy in mice. Specifically, the experiments proposed will test the effect of deletion of the NR2B subunit specifically in adult-born GCs on DG physiology, contextual fear learning and behavioral response to antidepressants.

DESCRIPTION (provided by applicant): My career goal is to train graduate, undergraduate, and medical students, and independently lead a research group that investigates the neural circuitry underlying motivated and mood-related behavior to eventually use this insight for translational research. To achieve this goal, I am proposing a project that provides me with significant training by examining how the dentate gyrus (DG) circuit contributes to anxiety-like behavior. Specifically, I will examine whether the developmental origin or regional position of DG granule cells (GCs) dictates their contribution to emotional behavior. To test this, I will use optogenetic techniques to control the activity of mature and adult-born granule cells in the dorsal or ventral DG to determine their relative contribution to anxiety-like behavior. My primary expertise is in mouse behavior, molecular biology and mouse genetics. My career development plan will expand on this by providing me essential training in patch clamp electrophysiology, in vivo electrophysiology, and in vivo optogenetic neuromodulation during behavior. As my career goal is to lead a research group examining the circuits that underlie affective behavior, and how they go wrong in disease states, these skills are not only required, but also essential to my success in leading a well-rounded, independent research career. In addition, as my previous focus has been on basic research, I have proposed to expand my training in translational neuroscience, so that I may apply my research to successfully collaborate with clinicians. Research Project Identifying the circuit mechanisms that underlie anxiety and depression is of utmost importance for treating psychiatric illness. In this proposal, I will examine how the dentate gyrus (DG) contributes to anxiety-like behavior. While classically studied for its role in spatial learning, there is significant support for a role for the DG in emotional behavior, but the mechanism for this remains unknown. A potential mechanism derives from the observation that emotional state can influence the production of new granule cells (GCs) from stem cells located in the adult DG. In addition, recent studies suggest the hippocampus is functionally segregated along its dorsal-ventral axis, influencing anxiety-like behavior through its ventral pole. This would suggest that GCs represent a functionally heterogeneous pool of neurons determined locally by their developmental origin and regionally by their position along the dorsal-ventral axi of the hippocampus. To test these possibilities, we have selectively expressed the blue light activated cation channel channelrhodopsin-2 (ChR2) and the yellow light activated chloride pump halorhodopsin (eNpHR3.0) in populations of mature and adult-born GCs. Using local circuit mapping in vitro, we will test the hypothesis that adult-born GCs modulate DG output. In vivo, we will test the hypothesis that optical stimulation or inhibition of GCs in the ventral DG preferentially influences anxiety-like behavior, while the dorsal DG impacts spatial learning. Finally, we will dissect the preferential contribution of adult-born GCs to anxiety-like behavior.

? DESCRIPTION (provided by applicant): Anxiety disorders such as posttraumatic stress disorder, generalized anxiety disorder, and panic disorder afflict almost one third of Americans with significant personal, financial and societal costs. Thus, identification of the brain circuits that are disordered in these illnesses would have a major benefit to society. In recent years, it has become appreciated that the hippocampus, in addition to its role in learning and memory, plays an important role in emotional behavior. We have recently shown that acute modulation of neuronal activity in the ventral hippocampus (vHPC) can powerfully and reversibly suppress innate anxiety-related behavior. However, the mechanisms by which the vHPC may control fear, anxiety, and negatively motivated behaviors have remained elusive. In this proposal, we will leverage optical techniques to interrogate the extended vHPC circuit to determine how it may control emotional behavior. We will focus on deconstructing the output from the vHPC to the hypothalamus and amygdala, two subcortical structures involved in the autonomic, neuroendocrine and motivational responses to emotionally charged stimuli. We will first map the extended vHPC network that controls emotional behavior, determining the cell-types types modulated by the vHPC, as well as the behavioral stimuli that recruit these output streams. Then, we will use projection-specific, optical modulation of vHPC terminals in the amygdala and hypothalamus to determine whether vHPC output streams are functionally dissociable in their contribution to innate anxiety, aversion and learned fear. Finally, we will use cell-type specific functional imaging to determine whether there exists a neuronal signature for anxiety state within the activity of populations of vHPC neurons. The ultimate goal of these studies is to develop a functional map of the extended vHPC circuit that modulates innate and learned fear, providing a novel target for therapies aimed at the treatment of these debilitating anxiety disorders.

Project summary The dentate gyrus subfield of the adult hippocampal formation exhibits a unique form of plasticity, the ability to generate new neurons throughout life. These adult born granule cells (abGCs) integrate into existing circuitry and have distinct properties during specific phases of their development. This process of adult neurogenesis is dynamically regulated by environment and emotional state; interventions that have negative effects on mood, such as stress and isolation can reduce levels of neurogenesis, while interventions that increase mood, such as enrichment and exercise can increase levels of neurogenesis. Behaviorally, these abGCs have been implicated in cognitive functions such as learning and memory, as well as mood related functions such as responses to stress or antidepressant treatment. However, we lack a complete understanding of the firing patterns of abGCs in freely behaving mice, how their dynamic encoding patterns differ from mature GCs (mGCs) and how environment may alter these activity patterns in vivo. Here we will address these long- standing questions in the field. Using novel, cutting-edge tools for monitoring and manipulating the activity of age-matched cohorts of abGCs in vivo, we will ask how these neurons contribute to hippocampal function during their development. First, we will use cell-type specific optical techniques to silence specific cohorts of these neurons during phases of context encoding and discrimination. Then we will use functional calcium imaging in freely moving mice to determine how abGCs function during these tests of context encoding and differentiation. Finally, we will determine how environmental enrichment and exercise alter the firing patterns and encoding properties of abGCs and the DG- CA3 circuit in vivo. The goal of these studies is to understand the mechanisms by which these neurons encode contextual representations during fear memory formation. Understanding the role of abGCs in this process may allow us to harness this unique form of plasticity in the adult brain for the treatment of fear and anxiety- related disorders.

The hippocampus (HPC) and prefrontal cortex (PFC) are implicated in anxiety disorders. In rodents, the ventral HPC (vHPC) and medial PFC (mPFC) form a circuit that regulates anxiety-related behavior in the elevated plus maze (EPM). The vHPC and mPFC synchronize in the theta-frequency (4-12 Hz) range during exploration of anxiogenic regions, and disrupting communication between the vHPC and mPFC prevents mice from avoiding the anxiety-provoking open arms of the EPM. Here we propose to reveal the detailed circuit interactions between the vHPC and mPFC that are crucial for anxiety-related behavior. In particular, our preliminary data has shown that populations of vHPC neurons are recruited during exploration of the open arms in the EPM, and inhibiting vHPC neurons disrupts open arm avoidance. However it is not known whether certain classes of mPFC neurons have preferential access to anxiety-related input from the vHPC. To answer this question, we will study vHPC neurons which project to specific classes of neurons in the mPFC, as mice explore the EPM. We will also study mPFC neurons which project to specific targets, e.g., the basolateral amygdala, to determine how they encode anxiety-related information. We hypothesize that classes of mPFC projection neurons which receive anxiety-related input from the vHPC will also encode anxiety-related information. Finally, we will study how prefrontal interneurons respond to theta-frequency input from the vHPC and regulate the anxiety-related responses of mPFC projection neurons. Our preliminary studies have shown that specific inhibitory neurons in the mPFC regulate prefrontal responses to vHPC input and contribute to anxiety-related avoidance. Here, we will test the hypothesis that in the EPM, theta-frequency input from the vHPC recruits these interneurons, thereby enhancing prefrontal responses to anxiety-related input from the vHPC and anxiety-related avoidance. Specifically, we will examine whether prefrontal inhibitory neurons synchronize to theta-frequency input from the vHPC during exploration of the EPM. Finally, we will examine how prefrontal interneurons regulate anxiety-related activity within specific classes of mPFC projection neurons. Together, these studies will elucidate cell-type specific and circuit-level mechanisms underlying the role of hippocampal- prefrontal networks in a commonly studied anxiety behavior. They will also answer general questions about how complex brain circuits operate. E.g., can one source of input differentially transmit emotionally-relevant information to various neuronal subtypes in a downstream target? How do inhibitory interneurons organize their activity in response to a rhythmic input? Do interneurons regulate emotional representations selectively, within specific projection neurons, or nonspecifically, across a network? The answers to these questions will reveal novel targets for disrupting pathological brain states associated with anxiety disorders.

PROJECT SUMMARY Oligodendrocyte precursor cells continually produce myelinating oligodendrocytes in the adult brain throughout life. Active myelination of adult brain circuits has been shown to be important for some forms of learning, and recent work from our groups has shown that this a crucial process in memory storage and retrieval. However, while previous work has provided essential insight into the regulation of myelin plasticity in the adult brain, it is not clear how this process impacts the dynamic nature of neural encoding within memory circuits. Myelination increases conduction velocity across individual axons, however how this translates to computations at the level of neural circuits and their subsequent behavioral outputs is poorly understood. Thus, a considerable gap exists between those findings related to axonal myelination in the adult and those that describe the neural coding dynamics that underlie memory encoding, consolidation and recall. In this proposal we aim to bridge this gap by 1) elucidating the cell types in the mouse medial prefrontal cortex that become myelinated after a learning experience, 2) determine how active myelination of cortical circuits impacts the cellular codes that support memory, 3) how activity-dependent myelination modulates the synchronization and interregional communication between the medial prefrontal cortex, amygdala and hippocampus during fear memory recall and 4) the temporal dynamics of oligodendrocyte precursor proliferation and differentiation in vivo after memory encoding. These studies will provide the first ever evidence for bidirectional interaction between new myelin formation and active memory encoding ensembles and will elucidate fundamental mechanisms of glial signaling during learning and memory.

Project Summary The brain transforms experiences into patterns of activity that control emotions, decisions and behaviors. Discriminating these patterns of activity allow these experiences to be stored as distinct entities, separating important stimuli from unimportant ones, and catalogued into memory. Modern techniques in neuroscience such as large-scale recording, computational tools for analysis of these datasets and circuit-based manipulations provide an opportunity for developing a deeper understanding the mechanisms of learning and discrimination. These efforts are of profound importance to human health, as the inability to encode experiences appropriate precision is a hallmark of cognitive disorders associated with aging. One way that neural circuits may discriminate experiences is by, at the population level, separating the neural representations of these experiences with learning, allowing a readout area to better decode the stimulus from the patterns of activity. A locus of this computation is the hippocampus (HPC), which, with learning, encodes the relationships and distinctions between behaviorally relevant variables. We have recently found the dentate gyrus subregion of the hippocampus classifies cortical representations of olfactory stimuli, increasing the distance between odor representations with learning. In this proposal, we aim to understand the mechanism by which stimulus representations in the DG change with learning. In Aim 1, we will use viral, electrophysiological and imaging tools to map the cell-types and networks that generate odor representations in the DG. In Aim 2 we will determine the local circuit mechanisms that control the flexibility of odor representations with learning, with a focus on dopamine-dependent modulation of encoding dynamics in DG GCs. In Aim 3, we will determine how aging impacts the flexibility neural representations in the DG, and how circuit-based manipulations can reverse neural discrimination deficits in aged mice. Understanding the mechanisms that support learning-induced flexibility of neural ensembles will facilitate the development of therapeutics for the treatment of age-related cognitive decline.