Hey-Kyoung Lee, Ph.D. - US grants

[unreadable] DESCRIPTION (provided by applicant): Loss of vision not only alters the function of the brain processing visual information, but also affects the function of other sensory systems. This type of "cross-modal" plasticity has been observed in blind humans, and is thought to provide a compensatory mechanism to better utilize the remaining sensory modalities in the absence of vision. While the cross-modal changes are beneficial to blind individuals, they pose a challenge when devising clinical interventions to overcome the loss of vision because extensive cross-modal changes in neural circuitry may hinder restoration of normal function. So far most research has focused on the systems level analyses of cross-modal changes, however, the cellular and molecular mechanisms have not been explored. The long-term objective of this application is to understand the cellular and molecular mechanisms underlying cortical plasticity following changes in visual experience. Recently we found that depriving vision (by dark-rearing) of rodents not only increases excitatory synaptic transmission in the superficial layers of the visual cortex, but also produces opposite changes in other primary sensory cortices. These changes followed the rules of a homeostatic plasticity mechanism, which provides stability to neural networks following prolonged perturbation in neural activity. These changes were accompanied by correlative changes in AMPA receptor subunit composition at synapses. We hypothesize that the homeostatic plasticity observed cross-modally in other sensory cortices by visual deprivation may be a cellular correlate of cross-modal plasticity observed in blind individuals. Interestingly, the homeostatic changes in the function of visual cortex, as well as other sensory cortices, by visual deprivation occurred quite rapidly (within a week) and were readily reversed by restoring vision (by re-exposing the animals to a lighted environment). In this proposal we will determine the cellular mechanisms and functions of global homeostatic cross-modal plasticity in primary sensory cortices. Specifically, we aim to investigate visual experience-induced global homeostatic plasticity in terms of its (1) induction mechanisms, (2) molecular mechanisms, and (3) functional consequences at a cortical circuit level. To do this, we will combine electrophysiological measure of excitatory synaptic transmission using whole-cell patch clamp techniques, biochemical and immunohistochemical analyses of synaptic proteins, and utilize various genetically altered mice and in vivo gene knockdown. Results from the proposed experiments will provide insights into developing better treatment options for various visual deficits, which may differ depending on the degree of vision affected and the extent of cross-modal changes elicited. PUBLIC HEALTH RELEVANCE It is known that blind individuals display a compensatory enhancement in the remaining sensations when compared to normal sighted individuals. These changes, termed "cross-modal plasticity", while beneficial to the blind individual, poses a challenge in developing effective treatments for vision loss because extensive cross-modal changes hinder restoration of normal function. Knowledge gained from our work will provide insights into developing better therapies for various forms of visual deficits, which may require distinct treatment options depending on the degree of vision affected and the extent of cross-modal changes elicited. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Proper integration of multiple senses is critical for providing a coherent perception of the environment. Cross-modal relay of sensory information is not only critical for multisensory integration, but provide substrates for cross-modal compensation when losing a sensory modality. For example, in blind individuals cross-modal compensation is observed as enhanced functionality in the remaining senses as well as recruitment of visual areas by Braille reading. While cross-modal plasticity is largely beneficial to blind individuals, it poses a challenge when attempting to recover function by clinical interventions. For instance, the success of recovering hearing by cochlear implants is reported to be inversely correlated with the extent of cross-modal sensory compensation. It is likely that recovery of vision will encounter similar obstacles. Most studies of cross-modal plasticity focuses on systems level analyses, hence cellular and circuit level mechanistic understanding is quite limited. We previously reported that depriving rodents of vision by dark-rearing not only alters synaptic transmission in primary visual cortex (V1), but also produces opposite changes in primary auditory cortex (A1). In particular, we observed an increase in excitatory synaptic transmission in the superficial layers of V1, but a decrease in excitatory synaptic transmission in the superficial layers of A1 following dark-rearing. While our data suggest that visual deprivation can globally alter excitatory synaptic transmission across different primary sensory cortices, it is unclear how these changes affect cortical function. In this proposal, we will test the hypothesis that visual deprivation-induced decrease in excitatory synaptic transmission alters the receptive field properties of A1 neurons. We will test our hypothesis by combining in vitro whole-cell recordings to assess specific circuit properties of layer 2/3 neurons in A1, and in vivo single unit recordings of these neurons from awake mice. Specifically, we will determine whether visual deprivation alters the functional circuitry (Aim 1) and the receptive field properties (Aim 2) of layer 2/3 neurons in A1. Results from our project will provide a cellular and circuit level mechanistic understanding of how loss of vision affects the functionality of A1 neurons. In addition, it will provide evidence that cross-modal changes occur in primary sensory cortices. PUBLIC HEALTH RELEVANCE: Blind individuals display enhanced functionality in the remaining senses, such as better sound localization and pitch discrimination. We will examine how loss of vision affects the function of primary auditory cortex, an area of the brain involved in sound processing. Results from our work will benefit the development of therapeutics for recovering vision, as extensive sensory compensation is known to impede the recovery of the lost sense.

DESCRIPTION (provided by applicant): Recent studies highlight that each primary sensory cortex does not work in isolation, but have some degree of interaction, which is not only critical for multisensory integration, but also important for sensory compensation in the event of losing a sensory modality. In blind individuals, there are several reports of cross- modal compensation that allow enhancement of the remaining senses. While cross-modal plasticity is largely beneficial to blind individuals, it hinders the recovery of function by clinical interventions. For example, the success of recovering speech recognition following cochlear implants is reported to inversely correlate with the extent of cross-modal plasticity. It is likely that similar obstacls will be met when trying to restore vision in blind. While there are many studies on cross-modal plasticity, most analyses are done at the level of systems neuroscience. Therefore, there is scarce information as to what types of changes happen at the cellular and circuit level. We previously showed that depriving rodents of vision increases the excitatory synaptic transmission in primary visual cortex (V1), in line with homeostatic adaptation. Importantly, we also found that visual deprivation reduces the excitatory synaptic transmission in the superficial layers of primary auditory cortex (A1). These results suggest that losing vision can cross-modally alter synaptic function in other primary sensory cortices, but how these cellular level changes alter the neuronal and circuit function of A1 is unknown. In the current proposal, we will test our hypothesis that visual deprivation-induced synaptic plasticity alters the functional circuitry and the neuronal receptive field properties in A1. To do this, we will determine whether visual deprivation alters the synaptic strength (Aim 1-1) and spatial extent (Aim 1-2) of specific excitatory and inhibitory circuitry of A1. To examine the in vivo consequences, we will examine whether visual deprivation alters the receptive field properties of neurons (Aim 2-1) and the population encoding in A1 (Aim 2-2). Results from our study will provide a comprehensive mechanistic understanding of how visual deprivation changes the functionality of A1. Functional connectivity across different brain regions is not restricted to sensory cortices. Therefore, our findings can be generalized to elucidate how neurons globally adjust to insults to other parts of the brain, such as would occur during neural injury, stroke and neurodegeneration.

? DESCRIPTION (provided by applicant): During a critical period of early postnatal development, an asymmetry in the quality of visual input to the two eyes shifts ocular preference away from the weaker eye and induces amblyopia, the most common cause of monocular visual deficits in humans. Amblyopia is highly resistant to reversal in adulthood, due in large part to th termination of the critical period of heightened plasticity. Understanding how the enhanced plasticity of the critical period is initiated and terminated over development is fundamental to th development of therapeutic strategies aimed to reactivate plasticity to treat amblyopia in adults, which can be translated to a clinical population and to other critical periods. A popular model for the regulation of the critical period proposes that inhibitory control of plasticity at excitatory synapses is mediated by the maturation of the output of fast-spiking interneurons (FS-INs) that mediate perisomatic inhibition. However, we have shown that ocular dominance plasticity can be induced several months after the maturation of perisomatic inhibition. We propose instead that ocular dominance plasticity is regulated by plasticity upstream of inhibitory output, likely affecting the recruitment of inhibition into functional circuits. In addition we propose that the functional connectivity of Pyr->FS synapses must be retained in a permissive range for ocular dominance plasticity to be expressed, as larger reductions in Pyr->FS connectivity induced by genetic manipulations inhibit the expression of ocular dominance plasticity. Our preliminary analysis of the regulation of excitation from pyramidal neurons onto FS-INs (Pyr->FS) reveals that monocular deprivation during the critical period may functionally disconnect FS-INs from the cortical network by significantly reducing the number of excitatory inputs onto these neurons. Therefore we hypothesize that a novel mechanism of plasticity, deprivation-induced loss of functional Pyr->FS connectivity 1) is an early and obligatory step in the shift in ocular dominance induced by MD, and 2) determines the timing of the critical period. We propose a multidisciplinary set of experiments to test these hypotheses that combine: the expertise of the Quinlan lab in the examination of physiological changes in visual cortex in vivo in response to monocular deprivation; the expertise of the Kirkwood lab in the direct assessment of contribution of changes in single synapses to activity-dependent plasticity in the visual cortex; and the expertise of the Lee lab in the use optogenetic methods to identify foci and mechanisms of activity-dependent changes in synaptic function. Our model for the regulation of the timing of the critical period refutes many widely-held assumptions regarding developmental changes in synaptic plasticity in the mammalian cortex.

? DESCRIPTION (provided by applicant): Long-term monocular deprivation (MD) initiated before any visual experience leads to permanent loss of vision through the occluded eye, and is much more resistant to recovery later in life compared to MD initiated after a period of normal vision. At a functional and anatomical level, it is known that long-term MD leads to a shift in ocular dominance (OD) in the primary visual cortex (V1). In particular, V1 neurons lose responses from the deprived eye, which are thought to result from mechanisms resembling long-term depression (LTD). While recent studies have reported recovery from long-term MD using either genetic manipulations or invasive pharmacological interventions, these cases have been limited to long-term MD after about a week of normal vision during early development. Studies have shown that MD initiated before eye opening in diverse animal models is much more resilient to recovery, and is thought to involve changes in thalamocortical (TC) inputs to V1. Hence methods to recover plasticity at TC synapses would benefit recovery from chronic long-term MD without initial vision. We recently found that deafening adult mice for a brief duration leads to potentiation of TC synapses in layer 4 (L4) of V1. Here we propose to examine whether deafening adult ferrets for a brief duration would allow cross-modal potentiation of TC synapses in V1 to promote recovery from chronic long-term MD initiated before eye opening. Ferret V1 is organized similar to humans with OD columns and orientation pinwheels, which overcomes the limits of mouse V1 lacking such modular organization. To test our hypothesis that deafening promotes TC plasticity in adult ferret V1, we will utilize channelrhodopsin based optogenetic tools to quantitatively measure the strength of TC synapses in L4 of V1 with or without deafening (Aim 1). In addition, intracortical synaptic strength in L4 will be quantified. T investigate whether cross-modal potentiation of TC synapses promotes recovery from chronic long-term MD, we will compare V1 neuronal functions, including OD and visual acuity, using multi-site laminar recording probes (Aim 2). The results from our study will determine whether cross-modal sensory deprivation in adults would promote recovery from chronic long-term MD by restoring TC plasticity. Our results could be generalized to promote recovery from sensory loss in other modalities, and pave a way to develop non-invasive means to recover from deprivation amblyopia in adults.

Project Summary It is well documented that the ability of the brain to undergo plasticity becomes limited in adults. In particular, sensory experience-dependent plasticity of cortical circuits is rather confined to a limited time during development, termed the critical period. Recovery and refinement of sensory processing is therefore difficult in adults. For example, the success rate of speech recognition in artificial cochlear implant patients becomes quite low, if the surgery is done later in life. Hence discovery of mechanisms that can recover adult cortical plasticity is of essence to benefit recovery of hearing or for treating abnormal auditory processing as occurs with tinnitus. We found that temporary visual deprivation is quite effective at producing large-scale plasticity in the adult primary auditory cortex (A1) of mice. Such changes occurred as potentiation of feedforward excitatory synapses from the primary auditory thalamus (MGBv) to layer 4 (L4) as well as L4 to L2/3. This was accompanied by weakening of synapses arising from lateral intracortical sources to L2/3 of A1. In parallel, we also observed refinement of cortical circuits of A1 L4 and L2/3. Collectively, these changes suggest that A1 circuit adapts to allow better processing of bottom-up auditory inputs, which is consistent with our published observation of refinement of A1 L4 neuronal receptive field and lowering of detection threshold in visually deprived mice. In this application, we aim to determine the mechanisms involved in driving adult A1 plasticity with visual deprivation, and whether visual deprivation improves auditory behavior in adults. Based on our observation that visual deprivation induced potentiation of thalamocortical (TC) inputs to A1 L4 requires audition, but no due to changes in the auditory environment, we surmise that there is central adaptation in circuits mediating auditory signals going through the thalamus and the cortex. In particular, we hypothesize that short-term visual deprivation promotes A1 plasticity in adults by regulating inhibitory circuits at the level of thalamus and cortex (Aim 1). The circuit and synaptic adaptation seen in A1 following vision loss accompanied refinement of A1 L4 neural function, and is predicted to enhance auditory function. We will examine how short- term visual deprivation alters auditory behavioral tasks in adults, and investigate whether this is due to changes in A1 neuronal responses and population encoding during auditory tasks using in vivo 2-photon imaging (Aim 2). Results from our proposed study will provide mechanistic understanding on how short-term visual deprivation enables plasticity of adult A1 via regulation of thalamic and cortical circuits, and will provide means to enhance auditory processing in the adult brain that could benefit development of treatment options for enhancing or recovering auditory function as would be needed for better prognosis of artificial cochlear implants. Furthermore, our results can be generalized to provide insights into how cortical circuits adapt to losing major inputs as it may happen during injury, stroke, and neuronal degeneration.

PROJECT SUMMARY Development of non-invasive tools for activating deep brain structures is critical for causally manipulating neural function in humans. Furthermore, such method, if able to elicit long-term plastic changes in neural circuits, will aid in functional recovery of neural function. One of the promising non-invasive neural modulation technique that has a potential to activate deep brain structures in a focal manner is ultrasound. Several groups have demonstrated that ultrasound can lead to neural activation, alter sensory responses, or cause behavioral outcomes. In this proposal, we aim to fill a gap in knowledge as to whether focal stimulation of deep brain structures using Low Intensity Focused Ultrasound (LIFU) leads to long-term changes in neural function relevant for functional recovery, especially in the adult brain with limited capacity for plasticity. It is known that the developmental loss of thalamocortical (TC) plasticity precedes the closure of the critical period for cortical plasticity in sensory cortices, which suggests that recovery of TC plasticity may be needed to restore plasticity in the adult brain. In line with this idea, several studies have reported that recovery of adult cortical plasticity is often accompanied by restoration of TC plasticity. A previous study demonstrated that patterned electrical stimulation of the visual thalamus (dLGN) produces long-term potentiation (LTP) of TC inputs to the primary visual cortex (V1) in adult rats. Here we will investigate whether non-invasive LIFU stimulation of dLGN can produce TC plasticity in adult V1. In Aim 1, we will determine whether LIFU stimulation leads to long-term plastic changes of dLGN inputs to layer 4 (L4) of adult V1. To do this, we will use a LIFU stimulation device developed by the Applied Physics Laboratory (APL) at Johns Hopkins University, and combine this with genetic tools to express exogenous genes specifically in activated dLGN neurons. Specifically, we will use genetic methods that can drive the expression of optogenetic tools (i.e. channelrhodopsin-2) selectively to LIFU-stimulated neurons to functionally assess long-term synaptic plasticity, and test the utility of a novel sonogenetic tool that can provide cell-type specificity to LIFU stimulation. In Aim 2, we will investigate whether LIFU stimulation can alter neural response properties of V1 L4 neurons using in vivo 2-photon Ca2+ imaging. Results from our study will determine whether LIFU stimulation can produce long- term plasticity of neural circuits in the adult brain, which can be relevant for designing non-invasive methods for functional recovery. At the very least, our study will provide genetic methodologies that can drive exogenous gene expression in deep brain structures using LIFU stimulation, and will provide information on whether sonogenetics can produce cell-type specific activation of deep brain structures.