Michael M. Halassa - US grants

[unreadable] DESCRIPTION (provided by applicant): Astrocytes release a number of gliotransmitters which modulate synaptic plasticity and neuronal excitability. Astrocytes activate neuronal NMDA receptors by releasing glutamate and D-serine in a calcium dependent fashion. Hyper-stimulation of the NMDA receptor results in excitotoxic neuronal death. Preliminary evidence indicates that status epilepticus (SE) results in a prolonged increase in astrocytic calcium excitability that is temporally correlated with SE-induced neuronal death. I will use electrophysiological techniques along with two-photon imaging to ask whether gliotransmission causes SE-induces neuronal death by stimulating the NMDA receptor. This will be facilitated by the use of astrocyte-specific inducible transgenic where inositol 1,4,5 triphosphate-dependent calcium signaling has been targeted. The role of astrocytes in brain disorders is poorly understood. The research propsed herein will give more insight to the role of astrocytes in epilepsy, and potentially, to many other brain disorders including Alzheimer's disease, Parkinson's disease, depression and schizophrenia. This knowledge will allow the scientific community to design novel treatments to target these disorders. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): In the mammalian brain, cortical disengagement from sensory processing is observed at multiple spatial and temporal scales. During active behavior, this process may alter routing of information relevant to selective attention, while during quiescence, it may be relevant for sleep stability and memory consolidation. Several lines of evidence suggest that cortical disengagement is mediated by thalamo-cortical dynamics, including spindle oscillations. Spindles are discrete 7- 15Hz cortical oscillations linked to activty of the thalamic reticular nucleus (TRN), a group of GABAergic cells surround the dorsal thalamus. Attenuated spindles are observed in schizophrenia, and may contribute to the sensory gating deficits observed in this disorder, while hypersynchronous spindles are thought to represent spike and wave discharges (SWDs) of absence epilepsy; the inappropriate expression of sensory disengagement during active waking. Despite their discovery seven decades ago, the basic phenomenology of spindles is undergoing major revision. While surface electroencephalographic (EEG) recordings in humans and local field potential (LFP) recordings in anesthetized animals have shown spindles to be coherent across cortical areas, recent human magnetoencephalographic (MEG) and implanted electrode recordings have revealed local expression of these events, suggesting that spindles have a local computational value linked to their roles in sensory filtering and memory. Using newly developed light-weight multi-electrode microdrives, I will record and manipulate electrophysiological activity across multiple sectors of the TRN in freely behaving mice. I will first refine an optogenetic approach that I have been using to determine the parameters under which local, modality-specific, control of TRN and related neocortex can be controlled (Aim I). In Aim II, I will use these parameters to causally control spindle generation and explore whether spindle type is dependent on the locus of TRN induction. In Aim III, I will test whether spindle expression attenuates sensory input in a modality-specific manner using somatosensory stimulation. These aims will directly test an important hypothesis about spindle expression and function, leading to greater insight into the pathogenesis of schizophrenia and absence seizures. In addition, insight into the principles by which thalamic firing modes contribute to routing of sensory information will be relevant to designing neural prosthetics for augmenting sensory function and cognition. Importantly, this proposal will allow me to learn optogenetic, electrophysiological, and behavioral techniques in mice, under the mentorship of Drs. Christopher Moore and Matthew Wilson. I will learn statistical and analytic techniques under the mentorship of Dr. Emery Brown. My future career goal is to combine my clinical experience with rodent studies to lead a translational research program that transcends species boundaries. I will use the human model to look for electrophysiological endophenotypes of neuropsychiatric disorders, and the rodent model to perform circuit-level dissection of these processes under physiological conditions and in models of disease. PUBLIC HEALTH RELEVANCE: This project will investigate how a certain type of cortical oscillation, spindles, are generated by thalamic mechanisms, and what functional significance their expression has on sleep and sensory function. Spindles are thought to be important for sleep stability, sensory filtering during sleep and sleep-dependent memory consolidation. Attenuated spindles are thought to contribute to the pathophysiology of schizophrenia, while excess spindles underlie the generation of spike and wave discharges of absence seizures. The proposed research, therefore, will have broad implications in understanding disease mechanisms and approach to rational correction.

? DESCRIPTION (provided by applicant): How information is routed to the neocortex as a function of behavioral state is a fundamental but poorly understood question in modern neuroscience. The thalamic reticular nucleus (TRN) is an inhibitory structure that is hypothesized to gate information throughput from thalamus to cortex, but experimental evidence clarifying the details of this process is lacking. To overcome these limitations, interrogate intac TRN microcircuits and understand their role in behavior we will create novel molecular tools that confer genetic targeting specificity to this brain structure Aim I. Using innovative genome editing technologies, we will maximize targeting success, while minimizing the time taken to validate these tools. Using these tools, we will ask how TRN microcircuits are functionally organized addressing how the thalamus may differentially gate multiple competing inputs to guide behavioral output, Aim II. In addition to opening several venues for further basic understanding of brain microcircuits, state regulation and cognition, this research will be of tremendous translational impact. In a parallel collaboration between Halassa and Feng, we are studying a mouse model of monogenic human autism with a primary TRN dysfunction. The genetic methods we are creating will equip us with the ability to develop molecular and optical interventional tools for treatment of this disorder and other neurodevelopmental diseases.

? DESCRIPTION (provided by applicant): Understanding the mechanisms of top-down attentional control is one of the most important endeavors in modern neuroscience. This process allows the brain to flexibly switch among processing different information streams and to extract relevant signals from equally salient noise. Top-down attention is critically disrupted n autism, schizophrenia and ADHD, and understanding its underlying mechanisms is therefore of great translational importance. While primate studies have established cortical substrates for top-down attention, our data using the mouse have revealed an unsuspected role for thalamic circuitry in this process. Specifically, we have observed rate and temporal modulation of the thalamic reticular nucleus (TRN), the major source of thalamic inhibition, in a top down attentional task. Disrupting this process diminishes task performance, suggesting causal dependency. Here, we will test the hypothesis that the TRN functions as a cognitive searchlight, translating top- down cortical input to changes in thalamic processing critical for behavioral outcome. In addition, we will investigate whether a disrupted searchlight explains distractibility and attentional impairment in a mouse model engineered to mimic a human autism variant. Our work will be enabled by a top-down attentional task we developed in mice, where animals switch between processing two sensory inputs on a trial-by-trial basis. In Aim I, we will combine multi-electrode recordings in TRN and optogenetic manipulations in prefrontal cortex, asking whether TRN attentional modulation is dependent on prefrontal top-down input. Using closed-loop optogenetic manipulations that distinguish between rate and temporal coding regimes, we will ask how TRN neural codes map onto behavioral outcomes. In Aim II, we will examine two putative mechanisms that couple TRN activity changes to downstream circuitry and behavior. For the first, we will develop a fiber photometry approach to measures dynamic changes in intracellular chloride, a proxy for synaptic inhibition. For the second, we will use a multi- electrode approach to infer dynamic changes in thalamo-cortical transmission. In Aim III, we will perform translational studies using the PTCHD1 knockout, a mouse engineered to mimic a human autism variant. PTCHD1 expression is selective to TRN in development, and we will test the relationship between diminished TRN burst generation and behavioral distractibility in the knockout. We will ask whether reversing TRN dysfunction rescues its behavioral distractibility. Because diminished top-down attention is a feature of several brain disorders, our therapeutic development will be of broad translational appeal. Overall, by providing deep insights into the circuit mechanisms of cognitive function, we aim to develop novel diagnostics and therapeutics for disorders of cognition.

PROJECT SUMMARY/ABSTRACT Sensory abnormalities characterize a wide range of neurodevelopmental disorders. In autism spectrum disorder (ASD), for example, sensory overload is one of the most frequently reported symptoms. Abnormal regulation of sensory information flow (sensory gating) is also observed in schizophrenia and ADHD, and is thought to contribute to overall cognitive dysfunction across all these conditions. Despite its central importance, little is known about the neurobiology of sensory gating, and even less is known about its failure in disease. This proposal aims to address this critical gap. The neocortex is requires for higher level sensory processing, but early processing and transmission of sensory information is performed by the thalamus. We and others have found that thalamic sensory input is controlled by the thalamic reticular nucleus (TRN), a shell of GABAergic neurons surrounding thalamic relay nuclei. The TRN is composed of individual subnetworks, each controlling thalamic flow in a modality-specific manner. Recent clinical data have shown thalamic and TRN dysfunction in neurodevelopmental disorders. Given the critical role for TRN in sensory processing, we expect perturbations in its circuits to pathologically augment cortical sensory input, explaining several clinical symptoms. In sleep, TRN dysfunction may result in increased sensory-related arousals, while in attention irrelevant inputs may become much more distracting. As such, a `leaky thalamus' may have profound consequences on behavior and cognition across disorders. In this proposal, we will test the leaky thalamus framework by manipulating thalamic inhibition in mice while monitoring the impact on sensory function and related behaviors. In addition, we will investigate the therapeutic potential of reversing thalamic inhibition deficits in models of human neurodevelopmental disorders.

PROJECT SUMMARY The thalamic reticular nucleus (TRN), the major source of thalamic inhibition, plays essential roles in sensory processing, arousal and cognition. Receiving inputs from cortical and subcortical regions, this structure is strategically positioned to influence thalamo-cortical interactions. During quiescence, the TRN participate in sleep rhythm generation, sleep stability and memory consolidation, while in active states, TRN neurons contribute to sensory filtering underlying attention. Perturbed TRN function may underlie behavioral deficits in disorders ranging from schizophrenia and autism to ADHD. Despite its importance, however, several key challenges have limited our ability to determine exactly how TRN circuitry contributes to various brain functions, a prerequisite for determining how it malfunctions in diseases and how its circuitry can be leveraged for diagnostic and therapeutic purposes. This proposal aims to address this critical gap in knowledge by capitalizing on a novel set of findings and tools that we generated. The TRN is a thin shell of GABAergic neurons surrounding thalamic projection nuclei. Within the TRN, neurons that have distinct structural and functional properties can be partially intermingled. This anatomical feature has been a major impediment for functional studies, since selective targeting of TRN neurons that share structural and functional properties with traditional methods is challenging. Using single cell RNAseq, we have recently discovered that TRN neurons can be dissociated into two major subtypes with distinct transcriptomic profiles, anatomical localizations, electrophysiological properties and thalamic connectivity. One group, located in the ?core? region of the TRN and can be marked by the expression of the Spp1 gene, targets first-order sensory thalamic nuclei, and the other, located in the ?shell? region of the TRN and marked by the expression of Ecel1 gene, targets higher- order ones. We have generated transgenic mice expressing Cre recombinase in each of these two populations individually. Here, we propose to use these new knowledge and genetic tools to answer fundamental questions about TRN structure-function organization as well as the contribution of this brain region to sensory processing, arousal and cognition.

Abstract Interactions between the cortex and thalamus are essential for sensation, action and cognition. Although the role of the thalamus in sensory processing is well-studied, its role in cognition is just beginning to be elucidated. This proposal will focus on the mediodorsal thalamus (MD), one of the largest thalamic nuclei of the mammalian brain, in regulating prefrontal cortex (PFC) activity in cognitive control and flexibility. MD-PFC interactions are known to be perturbed in schizophrenia, but therapeutic options are limited because the circuit mechanisms underlying these interactions are unknown. In this proposal, we will test an overarching model that the MD regulates PFC connectivity patterns to match the behavioral context through enhancement of context-relevant activity patterns and suppression of context-irrelevant ones. Specifically, our model posits that MD neurons generate context-specific signals by selectively strengthening PFC inputs that carry the basic elements of these signals but do not encode the context explicitly. Two MD cell types appear to generate such signals, one of which uses it to enhance context-congruent PFC activity while the other suppresses context incongruent activity. Lastly, our model posits that these distinct thalamic effects are mediated by different PFC circuit motifs, an excitatory/disinhibitory and an inhibitory, respectively. In Aim I, we will test the hypothesis that MD neurons generate context-specific output based on context non-specific inputs from PFC. In Aim II, we will test the hypothesis that the two MD functional cell types map onto distinct genetic types. In Aim III, we will test the hypothesis that the two MD-dependent effects on PFC, enhancement and suppression of activity patterns, are implemented by distinct local circuit motifs. Overall, our work should clarify the circuit mechanisms by which the MD influences PFC activity, providing a starting point for examining their generality across different task switching paradigms as well as relationship to higher order thalamus function more broadly. Our work should also be relevant to central mission of the NIH in understanding mechanisms with therapeutic potential.