Jennifer L. Raymond - US grants

The vetibulo-ocular refle (VOR) reduces motion of images on the retina by evoking eye movements in the opposite direction from head movement. Motor learning calibrates the VOR gradually correcting the reflex whenever image motion is persistently associate with heat turns. In addition, motor learning in the VOR in the VOR depends critically on the function of the cerebellum, and the principles uncovered in studies of the VOR may apply generally to many motor systems that are thought to rely on cerebellum- dependent learning to maintain normal sensory-motor function and for recovery of function following brain damage. The investigator proposes to develop a model system that would take advantage of the genetic tools as well the pharmacological and surgical manipulations available in the mouse to study motor learning in the VOR. Gene knockout mice will be used to more directly link cellular pathways to cerebellar physiology and physiology to motor learning in the VOR. This application aims to perform the necessary preliminary description of the basic behavior and physiology of the VOR in normal mice and to begin to use some interesting cerebellar mutants that are available to analyze the neural mechanisms for the induction of learning.

The vestibulo-ocular reflex (VOR) reduces motion of visual images on the retina by evoking eye movements in the opposite direction to head movements. A form of motor learning, known as VOR adaptation, calibrates the VOR by gradually correcting the reflex when image motion is persistently associated with head turns. VOR adaptation is essential for ensuring adequate visual acuity during head turns and for restoring proper motor and perceptual orientation in space in response to changes in the organism or its environment, such as occur with growth and development, aging, injury to the peripheral or central nervous system, the donning of a new pair of spectacles, or travel in space. The proposed experiments examine the neural mechanisms of VOR adaptation by asking the following questions: How do the parameters of visual-vestibular stimulation (image motion during head turns) affect VOR adaptation? (Aim 1) What patterns of neural signals are consistently present during stimuli that induce VOR adaptation, and might therefore serve as the neural trigger for VOR adaptation? (Aim 2) How might plasticity in neural pathways through the cerebellum contribute to VOR adaptation? (Aim 3) The VOR is one of many motor systems that is thought to rely on cerebellum-dependent learning to maintain normal sensorimotor function and for recovery of function following injury. The anatomy and physiology of the cerebellum is very regular across the extent of this structure, therefore, the principles uncovered in studies of VOR adaptation may be useful for the development of rational therapeutic approaches to many forms of sensorimotor dysfunction.

DESCRIPTION (provided by applicant): This project is the initial phase of a systematic research program for the functional dissection of a neural circuit. The experimental approach employs some of the latest transgenic techniques to rapidly and reversibly inactivate distinct classes of interneurons and analyzes the impact of this intervention on both signaling in the neural circuit and behavior. This strategy should yield a deeper and more comprehensive understanding of how a circuit's architecture shapes neural computations. The proposed experiments focus specifically on analysis of neural circuit function in the cerebellum, a brain structure that plays a central role in motor learning and the coordination of movements. The first Aim of the project is to generate transgenic animals that can be used to rapidly and reversibly inactivate a class of cerebellar interneurons called Golgi cells in a restricted region of the cerebellum, without perturbing any other neurons in the brain. The second Aim of the project is to use the mice generated in the first Aim to analyze the contribution of the Golgi cells to the neural computations supporting the generation of movements with appropriate amplitude and timing. These experiments represent the first step in a systematic analysis of how the different classes of neurons in the cerebellum and other neural circuits support the computational functions supporting perception and action. Many diseases of the nervous system involve the malfunction of neural networks caused by the degeneration of specific classes of neurons. This proposed research develops an experimental approach for analyzing how the function of neural networks is disrupted by the loss of specific populations of neurons. The results will aid in the development of rational therapeutic interventions for pathological states of the nervous system resulting from the degeneration of specific classes of neurons.

DESCRIPTION (provided by applicant): Motor learning is the process by which movements become smooth and accurate through practice. Motor learning is important during early childhood development, and continues throughout adulthood, because the neural circuits controlling our movements need to be recalibrated in response to changes in the brain or body due to injury, disease, or the normal aging process. Motor learning depends on a brain region called the cerebellum, and patients with cerebellar dysfunction have clumsy, uncoordinated movements. One of the two main inputs to the cerebellum is the "climbing fiber" input from the inferior olive in the brainstem. An influential theory of cerebellar function suggested that the climbing fibers carry the error signals that control motor learning. However, recent evidence suggests that motor learning can occur in the absence of instructive signals in the climbing fibers. Thus, there seems to be more than one way to implement motor learning in the brain. The goal of this project is to determine which aspects of motor learning are controlled by the activity of the climbing fibers, and which aspects of learning rely on other neural mechanisms. This question will be addressed by studying the eye movement responses to vestibular stimuli (i.e., the sensory signals encoding movements of the head) and their regulation by motor learning. Most, if not all, movements are guided by vestibular signals. The eye movement response to a vestibular stimulus is called the vestibulo-ocular reflex (VOR). This vestibular reflex functions to stabilize visual images on the retina, and is essential for maintaining good vision during movements of the body. Both the amplitude and the timing of the eye movements driven by the VOR can be adaptively modified by cerebellum-dependent learning, and thus the VOR serves as a model system for studying the neural mechanisms controlling movement amplitude and timing more generally. PUBLIC HEALTH RELEVANCE: An important characteristic of neural circuits is their plasticity, their ability to change with experience and to compensate when injury or disease damages the nervous system. This project studies the error signals that guide the changes in a neural circuit during learning. An improved understanding of this process will inform the development of more effective interventions for a broad range of neurological and psychiatric disorders.

? DESCRIPTION (provided by applicant): A high-resolution, low-cost, and easy-to-implement technique for measuring eye movements will be developed. This will accelerate the pace of research on vision by allowing widespread integration of eye movement measurements into increasingly sophisticated studies of visually- guided behavior, including studies in freely moving mice. Historically, research on sensory processing has focused to a great extent on the visual system. However, as neuroscientists turn increasingly to using mice in their research, they are turning increasingly away from the visual system in favor of other sensory systems. This is limiting the extent to which our understanding of the visual system can reap the benefits of the powerful molecular-genetic tools available in mice for analyzing neural circuits in health and disease. Mice are sometimes misperceived as being not very visual. Although less dependent on vision than some species, mice clearly use vision for a variety of functions, including navigation and evasion of predators. Thus, there is great potential for the molecular-genetic approaches available uniquely in mice to advance our understanding of the fundamental visual processing operations supporting such visually-guided behavior. Nevertheless a technical barrier is limiting studies of vision in mice, namely the difficulty of measuring and controlling ee movements and hence knowing what the input was to the visual system at the level of the retina. This technical barrier will be eliminated by developing a novel technique for measuring eye movements in mice, based on magnetic sensing. A small but powerful neodymium magnet will be used to create a rotating external magnetic field as the eye rotates in its socket. The angle of the magnetic field will then be detected by an external magnetic field sensor. To optimize performance of the system for measuring eye movements in mice, various magnet shapes, magnetic sensors, and geometric configurations of magnet and sensor will be rigorously tested. Performance will be compared against existing methods for measuring eye movements, namely video-oculography and the scleral search coil techniques. The magnetic system will then be assessed in mice that are free to move, and will be used to examine the coordination of eye and head movements during natural behaviors. Importantly, the techniques developed in mice will be readily adaptable to a broad range of species.

ABSTRACT OF PROPOSED SUPPLEMENT We will characterize cerebellum-dependent learning and memory in the 5XFAD mouse model of Alzheimer?s disease (Oakley et al., 2006), and compare impairments in these mice on different time scales: short-term (seconds, single trial), intermediate-term (tens of minutes, cumulative learning over a single training session) and long-term (days, consolidation). The Alzheimer?s mouse model can help achieve the parent grant?s scientific goal of dissecting the mechanistic relationship between different time scales of cerebellum-dependent learning. Moreover, there is evidence to suggest a role of the cerebellum in Alzheimer?s disease (Larner, 1997; Baloyannis et al., 2000; Ciavardelli et al., 2010; Mavroudis et al., 2010; Lomoio et al., 2012; Hoxha, et al. 2012; Baloyannis et al., 2013; Sepulveda-Falla et al., 2014; Kuwabara et al., 2014; Jacobs et al., 2017), but this is a highly understudied aspect of the disease compared to the role of the forebrain. The oculomotor system provides a sensitive and analytically tractable approach to understand broader learning and memory deficits in Alzheimer?s.

PROJECT SUMMARY Vision is an active sense, with eye movements powerfully shaping the acquisition of visual information about the world. This project investigates how motor learning adjusts the neural circuitry controlling eye movements, to maintain the accuracy of eye movements over short and long time scales. The specific focus of the research is to understand how oculomotor learning, i.e., improvement of the accuracy of eye movements through experience, is transferred from short-term to long-term storage. This consolidation process occurs, not only for motor skills like eye movements, but is a general feature of learning and memory systems. Some memories, including oculomotor memories, are transformed during the time after the initial acquisition of the memory, in a way that renders older, consolidated memories independent of brain areas that are critical for newer memories. This process, known as systems consolidation, is thought to depend on activity of neurons in the brain area initially critical for the memory, and the hypothesis is that this activity induces changes in the brain area(s) supporting long-term storage of the memory. The proposed research characterizes the nature of the neural signals transmitted between brain areas supporting memory at different times after their acquisition, and the rules that operate on those neural signals to implement stable transfer of a memory from one brain area to another.

PROJECT SUMMARY The goal of the proposed research is to implement state-of-the-art techniques for recording and manipulating neurons based on their activity in the cerebellum, to dissect the computations performed by the cerebellum to control eye movements. Vision is an active sense, and the accurate control of eye movements plays an essential role in vision. The cerebellum plays a key role in the control of eye movements, and in the refinement of eye movement accuracy and precision through oculomotor learning. It is known that the part of the cerebellum controlling eye movements receives visual and vestibular sensory information as well as copies of the eye movement commands, and presumably uses these sensory and motor signals to guide oculomotor performance and its modification by learning. However, a number of technical challenges have limited our ability to study how different sensory and motor signals are integrated in the cerebellum, and how the different signaling pathways are each modified during learning to improve oculomotor performance. Two newly developed tools, CaMPARI and Cal-Light, hold great promise to overcome some of the technical challenges that have limited studies of cerebellar computation. These tools offer advanced precision in the ability to record and manipulate neurons based on their activity during specific task conditions. We will (1) evaluate the efficacy of CAMPARI and Cal-light for selectively targeting (?tagging?), subpopulations of cerebellar neurons in a task- and activity-dependent manner, and (2) use these tools to dissect the computations implemented by the cerebellum during oculomotor performance and oculomotor skill learning. The technical outcome of the proposed work will be a new set of experimental approaches for studying the cerebellum, as well as new experimental strategies for studying computation and learning in other neural circuits. The scientific outcome will be new insights about the computations performed by the cerebellum on its sensory (visual and vestibular) and motor (efference copy) inputs. Advances in understanding how the cerebellum supports accurate eye movements will provide conceptual underpinning for developing more rational interventions for oculomotor disorders, and, more generally for the wide array of disorders associated with cerebellar dysfunction.