Aaron W. McGee - US grants

DESCRIPTION (provided by applicant): Nogo is a neuronal and myelin membrane protein that inhibits neurite outgrowth and impedes functional recovery after spinal cord injury. Examining the trafficking of nogo and the nogo-66 receptor (NgR) in neuronal cultures will address functions of nogo in regulating axon and dendritic spine formation. Mice lacking a functional gene for nogo display elongation and collateral sprouting of damaged axons, and improved locomotor activity following spinal cord injury (SCI). Understanding the physiological function(s) of nogo in contexts other than SCI may provide opportunities to develop rational interventions for promoting functional recovery from CNS injury. In mice lacking nogo, or NgR, analysis of afferent terminations from dorsal roots adjacent regions of the spinal cord denervated by rhizotomy will determine if nogo or NgR can increase the sprouting of uninjured neurons. The plasticity of the vibrissae (whisker) representations (barrels) in somatosensory cortex may be aberrant these mice. Comparing the pattern of barrels by staining, and neuronal structural plasticity with chronic imaging of cortical pyramidal neurons in these mutant mice may elucidate role of nogo and signaling through the NgR in regulating cortical plasticity.

DESCRIPTION (provided by applicant): Novel sensory experience and improved motor performance modify structural synaptic connectivity in the cerebral cortex, yet the functional contribution of this anatomical plasticity to perceptual learning is unclear. Recently, emerging techniques for repeatedly imaging neuronal structures in vivo has focused attention upon the remodeling of dendritic spines as a potential substrate for learning. However, a major obstacle to resolving the relationship between spine remodeling and learning is determining if spine dynamics are specific to cortical region, neuronal identity and learning task. The barrel field in somatosensory cortex (barrel cortex) has been a favored system for examining spine remodeling during sensory experience due to the topographical representation of whiskers and controlled, quantifiable nature of whisker use. The gap crossing task is an automated, quantitative perceptual learning task that relies on detection of a gap between two platforms by the whiskers. These experiments combine chronic in vivo imaging of dendritic spines in barrel cortex with this perceptual learning task to investigate if and how the rate of spine remodeling may specify the rate of perceptual learning. To test the hypothesis that the rate of spine remodeling reports the rate of perceptual learning, subsequent combined imaging and learning experiments exploit the phenotype of nogo-66 receptor (NgR1) mutant mice that learn this task faster. NgR1 is a neuronal protein that regulates plasticity in both the injured and intact central nervous system. NgR1 mutants recover better from spinal cord injury and stroke; adult NgR1 mutants also display a form of visual plasticity normally confined to a developmental critical period. How NgR1 regulates both spine remodeling and perceptual learning may improve not only understanding of how anatomical plasticity contributes to learning, but may reveal conserved mechanisms by which this receptor governs plasticity after injury and during development as well.

Abstract The visual system exhibits a heightened sensitivity to the quality visual experience during an interval late in development termed the `critical period'. Discordant vision during the critical period is the cause of amblyopia, a prevalent visual disorder in children. Treatment of amblyopia is most effective in children before the close of the critical period. Subsequently, the flexibility with brain circuitry diminishes in adulthood and effective therapy is more difficult. In a mouse model of amblyopia, disrupting normal vision by closing one (monocular deprivation, MD) for the duration of the critical period, but not thereafter, decreases visual acuity and perturbs the normal eye dominance of neurons in visual cortex. The nogo-66 receptor gene (ngr1) is required to close the critical period. In ngr1 mutant mice, plasticity during the critical period is normal, but it is retained in adult mice. Importantly, ngr1 mutant mice spontaneously recover visual acuity in this model of amblyopia. Our overall hypothesis is that recovery of acuity and eye dominance are independent. In the proposed research, we take advantage of this extended critical period in ngr1 mice to identify with location and mechanisms of plasticity that mediate recovery of acuity and eye dominance with a combination of conditional mouse genetics, behavioural assays, electrophysiology, sophisticated repeated in vivo calcium imaging and laser-scanning photostimulation circuit mapping. We will begin to unravel how plasticity within visual circuitry mediates recovery of visual function following early abnormal vision (MD), as well as how this plasticity is restricted to the critical period with these experiments. In addition to improving understanding of how experience-dependent plasticity changes the function of brain circuits, these studies may reveal new avenues for developing therapeutic approaches to treat amblyopia. .

PROJECT SUMMARY/ABSTRACT The visual system exhibits a heightened sensitivity to the quality visual experience during an interval late in development termed the critical period. Discordant vision during the critical period is the cause of amblyopia, a prevalent visual disorder in children. Treatment of amblyopia is most effective in children before the close of the critical period. Subsequently, the flexibility with brain circuitry diminishes in adulthood and effective therapy is more difficult. In a mouse model of amblyopia, disrupting normal vision by closing one eye for only a few days (monocular deprivation, MD) during the critical period, but not thereafter, also perturbs the normal binocularity of neurons in visual cortex and decreases visual acuity. Yet how these adaptive changes, or plasticity, first emerge within neurons that form the circuits in visual cortex is poorly understood. Likewise, how plasticity propagates from the first neurons to adapt to other neurons connected to these neurons by synapses is unclear. The short duration of the critical period in mice is one factor impeding the study of how the greater plasticity confined to the critical period contributes to the induction as well as recovery from amblyopia. The nogo-66 receptor gene (ngr1) is required to close the critical period. In ngr1 mutant mice, plasticity during the critical period is normal, but it is retained in adult mice. Importantly, ngr1 mutant mice spontaneously recover visual acuity in this model of amblyopia. In the proposed research, we take advantage of this extended critical period in ngr1 mice to investigate what is unique about plasticity during the critical period that promotes recovery from amblyopia. We compare how MD alters the function and connectivity of populations of neurons in visual cortex with a combination of sophisticated repeated in vivo calcium imaging and laser-scanning photostimulation synaptic mapping. We will begin to unravel how plasticity within visual cortex proceeds during abnormal vision (MD), as well as how this plasticity is restricted to the critical period with these experiments. In addition to improving understanding of how experience-dependent plasticity changes the function of brain circuits, these studies may reveal new avenues for developing therapeutic approaches to treat amblyopia and perhaps other neurodevelopmental disorders that result from maladaptive developmental plasticity.

Project Abstract Fragile X Syndrome (FXS) is a leading inheritable cause of mental impairment. There is no known cure for FXS or treatment that reverses the collective pathology. There is a fundamental gap in our knowledge of how FXS causes mental impairments through alteration of neural circuitry. The long-term goal of this research is to develop an understanding of FXS that links learning impairments to specific changes in neural circuits. Characteristic symptoms of FXS include reduced intellectual abilities, learning deficits, and hypersensitivity to sensory stimuli. FXS arises from a loss-of-function in the FMR1 gene; mice lacking a functional fmr1 gene exhibit several phenotypes similar to FXS. Fmr1 mutant mice display an intriguing deficit on both the gap cross task, a freely-behaving whisker-dependent tactile learning task, as well as a head-fixed whisker-dependent tactile learning task. The central hypothesis is that impairments in tactile learning are driven by reduced dendritic spine stability and hypersensitive touch responses in primary somatosensory cortex resulting from attenuated activity of somatostatin-expressing (SOM) interneurons. Experiments in this proposal will determine the extent to which loss of the fmr1 gene disrupts spine stability, tactile learning, and circuit dynamics during task performance. Guided by our strong preliminary data, we will pursue this hypothesis in two related specific aims. In Aim 1, longitudinal two-photon in vivo imaging is combined with an automated head-fixed whisker- dependent tactile learning task to evaluate if reduced activity of SOM interneurons in fmr1 mutant mice decreases dendritic spine stability and impairs learning. In Aim 2, sophisticated electrophysiology is combined with high-speed tracking of whisker position during this same head-fixed object localization to quantify the extent to which tactile discrimination and cortical representations of afferent sensory activity in somatosensory cortex are abnormal fmr1 mutant mice and if attenuated function of SOM interneurons contributes to this deficit. This approach is particularly innovative because the synaptic changes that underlie learning are measured longitudinally throughout task acquisition. Furthermore, breaking from the anesthetized status quo, the cortical circuit dynamics that represent touch are quantified during active perceptual behavior. The proposal is significant because it vertically advances our knowledge of FXS mechanisms across levels of analysis, from synapse to circuit to behavior. Additionally, it opens new horizons for these advanced techniques to be applied to other cortical layers and brain regions to build a comprehensive understanding of neural circuit defects in a premier FXS model system. This proposal squarely meets the key mission objectives of the NINDS and NIMH to provide detailed and integrated knowledge of how the function of synapses and circuits is disrupted in neurological disorders. Ultimately, the resulting improved understanding of circuit dysfunction has the potential to lead to therapies that improve the quality of life for the roughly 1 in 5,000 people born with Fragile X Syndrome.