Jason M. Christie - US grants

DESCRIPTION (provided by applicant): Neurons are likely the most complex cell in the body with differentiated structures including a soma, dendrites, and axons. This structural diversification allows for a specialized functionality within each of these neuronal elements. Electrical signals develop at synaptic input sites on the dendrite, are compiled at the soma, and are then transmitted to synaptic output sites on the axon as action potentials (APs) following initiation in the axon initial segment (AIS). In myelinated axons of projection (principal) neurons fast salutatory conduction ensures that the resulting APs are rapidly propagated to release sites in a stereotyped manner ensuring a reliable trigger for neurotransmission. Intuitively, the regenerative nature of AP propagation over long distances suggests that the influence of the AIS in determining spike waveform should be spatially differentiated from sites of release. In comparison, AP signaling in the unmyelinated axons of compact interneurons is poorly understood. We hypothesize that axons of interneurons are not exacting relay devices of the AIS, rather, that these processes are also endowed with a capacity to locally determine and sculpt AP waveforms and that this property is an important element in determining dynamics of neurotransmission. In this proposal, we will examine three key parameters that would define and support location-specific control of axonal electrogenesis in cerebellar stellate cell interneurons: (1) directly measure AP waveforms in axons, (2) relate these findings to axon morphology and to the organization of ion channels in axonal compartments, and (3) determine whether the location-specific distribution and properties of ion channels confers activity-dependent control of axonal excitation and release. In this way, this work aims to identify the characteristics that may enable compartmental organization of axonal electrogenesis in interneurons with the goal of relating the specific and dynamic parameters of axon physiology to information processing in neural circuits. This project will help inform the development of therapeutic strategies targeting diseases of axon dysfunction where differentiation of AP initiation, propagation, and release may be required to ameliorate pathological conditions specific to each of these functions.

Project Summary    A goal of basic mental health research is to understand the molecular, cellular and circuit level substrates that  contribute to neuropsychiatric disorders. The goal of this project is to better understand the principles  underlying circuit dysfunction associated with cognitive and social impairments common to these disorders. A  promising approach to better understand these substrates is to perform in-­depth studies in animal models with  high construct and face validities. De novo pathogenic SYNGAP1 mutations leading to haploinsufficiency  cause one the most common genetically defined and non-­inherited forms of intellectual disability (ID) with  autism spectrum disorder (ASD;? termed MRD5;? OMIN# 612621). Studies supported by the first budget period  identified Syngap1 heterozygous KO mice as an outstanding genetic model of ASD with ID. Using this model,  we discovered a developmental sensitive period of Syngap1 function that promotes the proper function of  cortical networks. The neurobiological studies we published in the last period were significant because they  identified the developmental timing of dendrite and spine maturation selectivity within forebrain excitatory  neurons as a critical substrate that shapes brain function relevant to cognitive and social development.     For this competitive renewal, we will build on our discoveries in the first budget period by studying the key  substrates of circuit dysfunction in the Syngap1 model by probing how this gene regulates cortical sensory  processing relevant to cognition and learning. This approach is significant because sensory impairments are  extremely common in ASD/ID and these impairments influence behavioral adaptations, including learning.  Syngap1 patients express sensory abnormalities related to touch and pain. However, the circuit abnormalities  that underlie sensory dysfunction are unclear. Thus, our approach is innovative because studies will be  performed in the mouse somatosensory cortex, which will enable powerful in vivo experiments that are capable  of directly linking cellular-­ and circuit-­level functional impairments to sensory-­based learning and behavioral  abnormalities. The first Aim will investigate the cellular mechanisms underlying impaired somatosensory cortex  network function caused by pathogenic Syngap1 mutations, with an emphasis on how network-­level E/I  imbalances emerge within cortical circuits that directly encode sensory representations. Research proposed in  the second Aim will determine the cellular mechanisms that contribute to sensory-­driven learning impairments  in Syngap1 mice. The impact of these studies is that they are expected to advance our understanding how  cortical circuit dysfunction leads to behavioral impairments associated with ASD/ID.    

Project Summary/Abstract My long-term goal is to understand how neural circuits in the cerebellum ensure accurate movement through the acquisition of motor learning. When cued by performance errors, climbing fiber excitation triggers a response in postsynaptic Purkinje cells that involves both a complex spike in their somata and calcium spikes in their dendrites. While climbing fibers are highly reliable at driving complex spikes in Purkinje cells, they are not always effective at inducing learning. This indicates that there are specific processes during behavior that regulate the conversion of climbing fiber activity into adaptive information for the circuit. By regulating climbing fiber-mediated learning, inappropriate motor associations may be rejected and/or allow for other instructive signals to engage mechanistically-distinct types of plasticity. Ultimately, these mechanisms could underlie a range of adaptive responses that vary in time, amplitude, and direction. In this proposal, we explore the possible role of molecular layer interneurons (MLIs) in regulating Purkinje cell excitation in response to climbing fiber activation, with special focus on dendritic Ca2+ spikes that are particularly relevant to known mechanisms of plasticity at coactive parallel fiber synapses. We will use quantitative measurements afforded by ex-vivo brain slice preparations to mechanistically dissect the interplay of climbing fiber excitation and ML inhibition at Purkinje cell dendrites, and the influence of these interactions on the climbing fiber?s ability to generate synaptic plasticity. In conjunction, we will use in vivo methods to measure and manipulate neural activity during the acquisition of adaptive motor responses in vestibulo-ocular system to gauge how learning is affected by MLI inhibition. Our study intersects cellular and systems approaches to link the cell-level signaling mechanisms to synaptic plasticity and the circuit processes that encode adaptive behavior. Because cerebellar pathology may manifest as inappropriate learning rather than the inability to learn, completion of these aims will provide novel insight into the development of new therapies to treat cerebellar disorders that affect motor control.

Project Summary/Abstract During motor learning, the cerebellum encodes memories of sensorimotor associations that predict deviant action and, during recall of these associations, it will impose adaptive changes to instill corrective behavior. This memory process depends on plasticity that alters the output of the cerebellum through learned patterns of Purkinje cell spike output. Molecular layer interneurons (MLIs) are excited by parallel fibers that convey sensorimotor information relayed through the mossy fiber pathway and, in turn, exert feedforward inhibition onto postsynaptic Purkinje cells to reduce their spike output. MLI synapses are plastic and therefore may be susceptible to learning-induced modification that would alter their inhibitory influence on Purkinje cells and, in this way, impart adaptive behavior. Yet, a basic understanding of how MLIs are affected by experience and if their activity is necessary for the expression of learning is unknown, creating a knowledge gap in the understanding of cerebellar function. Therefore, the objective of this study is to elucidate the role of MLIs in adaptive motor control in behaving mice and measure for learning-induced plasticity in their response properties. This will be accomplished in two aims. In the first, we will use electrophysiology and genetically encoded effectors of activity to measure and manipulate MLI responses in vivo during a motor-learning behavior: adaptation of the vestibulo-ocular reflex (VOR). This will allow us to determine if learning alters how MLIs are activated during sensorimotor stimulation and if their inhibitory output is necessary for pattern changes in Purkinje cell spiking and the expression of learned eye movements. In the second aim, quantitative measurements from cerebellar slice preparations of mice that gave undergone VOR learning will be used to determine if MLIs show activity- induced plasticity in their synaptic properties. This study encompasses an innovative, multidisciplinary approach to decipher the cellular- and circuit-level mechanisms that allow the cerebellum to encode memories of motor learning and implement adaptive motor behavior. Completion of these aims will contribute to novel insights into understanding how the cerebellum stores and recalls memories of learning.

Project Summary The project remains the same as the original application. Below is a summary overview. Our long-term goal is to generate a complete understanding of how the cerebellum learns to improve movement in response to motor errors. Climbing fibers are thought to play an essential role in this process because they fire during erroneous movement. Their activity reliably excites Purkinje cells, eliciting calcium spikes in their dendrites that can trigger long-term synaptic plasticity at coactive parallel fiber inputs. Plasticity induction ultimately leads to corrective behavior by altering the cerebellum's response to sensorimotor stimuli that predict mistakes. Importantly, inhibition from molecular layer interneurons (MLIs) that target Purkinje cell dendrites suppresses climbing-fiber-evoked calcium signaling, opposing or `gating' plasticity induction. Because MLIs are activated by movement, this suggests Purkinje cell disinhibition is required during motor learning. As MLIs inhibit other MLIs, their interconnections could support a circuit for Purkinje cell disinhibition during behavior. The objective of this proposal is to examine the possibility that MLI circuits are structured to support a context-dependent engagement that allows climbing fibers to instruct plasticity and learning in response to motor errors. To accomplish this, we will employ a multidisciplinary approach using cutting-edge molecular-genetic techniques, functional recordings, circuit mapping, and behavioral analysis. In the first aim, we will test whether ablating MLI-to-MLI connections that normally support Purkinje cell disinhibition affect the ability of climbing fibers to evoke full-blown calcium signals in response to motor errors, and whether loss of MLI-MLI circuit function affects cerebellar-dependent motor learning. In the second aim, we will establish an MLI taxonomy and use it to survey for previously unknown MLI subtypes. We will also use functional recordings to test whether there is evidence for bias connectivity within the MLI network that supports a dedicated circuit for Purkinje cell disinhibition. In the third aim, we will use anatomical tracing to ascertain the MLI connectome. In this way we will determine if there is a structural basis for the independent actuation of MLI subtypes through their afferent inputs and the cell-type selectivity of their efferent outputs. Completion of these aims will lead to an unprecedented understanding of the organizational logic of the molecular layer. In particular, we expect to reveal how circuits within the molecular layer control the induction of climbing-fiber- mediated learning. This knowledge will not only help develop theories/models of cerebellum function but will also provide insight into the processes underlying learning in general.