Aaron McGee

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• Brown, T. C., & McGee, A. W. (2025). Representational drift gates critical-period plasticity in mouse visual cortex. Current Biology. doi:10.1016/j.cub.2025.07.026
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  Brief monocular deprivation during a developmentally critical period, but not thereafter, shifts cortical responses toward the non-deprived eye. The characteristics of neural circuitry that permit this experience-dependent plasticity are poorly understood. Here, we performed repeated calcium imaging at cellular resolution to track the tuning properties of populations of excitatory layer 2/3 neurons in the visual cortex of juvenile mice during the critical period, adult mice after the critical period, and adult nogo-66 receptor (ngr1) mutant mice that retain critical-period plasticity. The instability of tuning for populations of neurons, termed “representational drift,” was significantly greater during the critical period than in adulthood. Adult ngr1 mutant mice displayed representational drift similar to that of juvenile mice. We propose that representational drift adapts the tuning of populations of neurons to recent experience during the critical period.
• Brown, T. C., & McGee, A. W. (2025). Representational drift gates critical-period plasticity in mouse visual cortex. Current biology : CB, 35(17), 4251-4258.e3.
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  Brief monocular deprivation during a developmentally critical period, but not thereafter, shifts cortical responses toward the non-deprived eye. The characteristics of neural circuitry that permit this experience-dependent plasticity are poorly understood. Here, we performed repeated calcium imaging at cellular resolution to track the tuning properties of populations of excitatory layer 2/3 neurons in the visual cortex of juvenile mice during the critical period, adult mice after the critical period, and adult nogo-66 receptor (ngr1) mutant mice that retain critical-period plasticity. The instability of tuning for populations of neurons, termed "representational drift," was significantly greater during the critical period than in adulthood. Adult ngr1 mutant mice displayed representational drift similar to that of juvenile mice. We propose that representational drift adapts the tuning of populations of neurons to recent experience during the critical period.
• Brown, T. C., Crouse, E. C., Attaway, C. A., Oakes, D. K., Minton, S. W., Borghuis, B. G., & McGee, A. W. (2024). Microglia are dispensable for experience-dependent refinement of mouse visual circuitry. Nature neuroscience, 27(8), 1462-1467.
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  To test the hypothesized crucial role of microglia in the developmental refinement of neural circuitry, we depleted microglia from mice of both sexes with PLX5622 and examined the experience-dependent maturation of visual circuitry and function. We assessed retinal function, receptive field tuning of visual cortex neurons, acuity and experience-dependent plasticity. None of these measurements detectibly differed in the absence of microglia, challenging the role of microglia in sculpting neural circuits.
• Brown, T., Crouse, E., Attaway, C., Oakes, D., Minton, S., Borghuis, B., & McGee, A. (2024). Microglia are dispensable for experience-dependent refinement of mouse visual circuitry. Nature Neuroscience, 27(8). doi:10.1038/s41593-024-01706-3
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  To test the hypothesized crucial role of microglia in the developmental refinement of neural circuitry, we depleted microglia from mice of both sexes with PLX5622 and examined the experience-dependent maturation of visual circuitry and function. We assessed retinal function, receptive field tuning of visual cortex neurons, acuity and experience-dependent plasticity. None of these measurements detectibly differed in the absence of microglia, challenging the role of microglia in sculpting neural circuits.
• Ma, X., Chen, P., Wei, J., Zhang, J., Chen, C., Zhao, H., Ferguson, D., McGee, A. W., Dai, Z., & Qiu, S. (2024). Protocol for Xenium spatial transcriptomics studies using fixed frozen mouse brain sections. STAR protocols, 5(4), 103420.
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  Here, we present a protocol for Xenium spatial transcriptomics studies using fixed frozen mouse brain sections. We describe steps for intracardiac perfusion, cryosectioning, and floating section mounting of brain sections, which enable runs on the Xenium analyzer and data delivery. We demonstrate that, in addition to the 10× Genomics-validated formalin-fixed paraffin-embedded (FFPE) and fresh frozen sections, fixed frozen thin brain sections are compatible with the Xenium platform and provide excellent imaging and quantification results for spatially resolved gene expression. For complete details on the use and execution of this protocol, please refer to Ma et al..
• Ma, X., Chen, P., Wei, J., Zhang, J., Chen, C., Zhao, H., Ferguson, D., McGee, A., Dai, Z., & Qiu, S. (2024). Protocol for Xenium spatial transcriptomics studies using fixed frozen mouse brain sections. STAR Protocols, 5(4). doi:10.1016/j.xpro.2024.103420
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  Here, we present a protocol for Xenium spatial transcriptomics studies using fixed frozen mouse brain sections. We describe steps for intracardiac perfusion, cryosectioning, and floating section mounting of brain sections, which enable runs on the Xenium analyzer and data delivery. We demonstrate that, in addition to the 10× Genomics-validated formalin-fixed paraffin-embedded (FFPE) and fresh frozen sections, fixed frozen thin brain sections are compatible with the Xenium platform and provide excellent imaging and quantification results for spatially resolved gene expression. For complete details on the use and execution of this protocol, please refer to Ma et al.1
• Walters, B. N., Whiddon, Z. D., McGee, A. W., & Krimm, R. F. (2024). Longitudinal imaging of the taste bud in vivo with two-photon laser scanning microscopy. PloS one, 19(12), e0309366.
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  Taste bud cells in the tongue transduce taste information from chemicals in food and transmit this information to gustatory neurons in the geniculate ganglion that innervate taste buds. The peripheral taste system is a dynamic environment where taste bud cells are continuously replaced, but further understanding of this phenomenon has been limited by the inability to directly observe this process. To overcome this challenge, we combined chronic in vivo two-photon laser scanning microscopy with genetic labeling of gustatory neurons and taste buds to observe how cells within the taste bud change over time. This method expands the investigative possibilities beyond those offered by fixed-tissue methods. This method permits direct observation of taste bud cell entry, cell differentiation, cell loss, and arbor plasticity. We demonstrate that a few stains/dyes can be used to observe nuclei and organelles in the taste bud in vivo. We also describe a workflow for reconstructing composite z-stacks with grayscale data of both cells and arbors using ImageJ, Neurolucida 360, and Neurolucida Explorer software. Together, the methodology and software options for analyses presented here provide a novel approach for longitudinally observing taste bud cells and arbors in the taste bud in vivo.
• Brown, T. C., Brown, T., McGee, A. W., & McGee, A. (2023). Monocular deprivation during the critical period alters neuronal tuning and the composition of visual circuitry. PLoS Biology, 21(4). doi:10.1371/journal.pbio.3002096
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  Abnormal visual experience during a developmental critical period degrades cortical responsiveness. Yet how experience-dependent plasticity alters the response properties of individual neurons and composition of visual circuitry is unclear. Here, we measured with calcium imaging in alert mice how monocular deprivation (MD) during the developmental critical period affects tuning for binocularity, orientation, and spatial frequency for neurons in primary visual cortex. MD of the contralateral eye did not uniformly shift ocular dominance (OD) of neurons towards the fellow ipsilateral eye but reduced the number of monocular contralateral neurons and increased the number of monocular ipsilateral neurons. MD also impaired matching of preferred orientation for binocular neurons and reduced the percentage of neurons responsive at most spatial frequencies for the deprived contralateral eye. Tracking the tuning properties for several hundred neurons before and after MD revealed that the shift in OD is complex and dynamic, with many previously monocular neurons becoming binocular and binocular neurons becoming monocular. Binocular neurons that became monocular were more likely to lose responsiveness to the deprived contralateral eye if they were better matched for orientation prior to deprivation. In addition, the composition of visual circuitry changed as population of neurons more responsive to the deprived eye were exchanged for neurons with tuning properties more similar to the network of responsive neurons altered by MD. Thus, plasticity during the critical period adapts to recent experience by both altering the tuning of responsive neurons and recruiting neurons with matching tuning properties.
• Frantz, M., Crouse, E., Sokhadze, G., Ikrar, T., Stephany, C., Nguyen, C., Xu, X., & McGee, A. (2023). Erratum: Layer 4 Gates Plasticity in Visual Cortex Independent of a Canonical Microcircuit (Current biology : CB (2020) 30 15 (2962-2973.e5) PII: S0960-9822(23)00670-X). Current biology : CB, 33(12). doi:10.1016/j.cub.2023.05.038
• Whiddon, Z., Marshall, J., Alston, D., McGee, A., & Krimm, R. (2023). Rapid structural remodeling of peripheral taste neurons is independent of taste cell turnover. PLoS Biology, 21(8). doi:10.1371/journal.pbio.3002271
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  Taste bud cells are constantly replaced in taste buds as old cells die and new cells migrate into the bud. The perception of taste relies on new taste bud cells integrating with existing neural circuitry, yet how these new cells connect with a taste ganglion neuron is unknown. Do taste ganglion neurons remodel to accommodate taste bud cell renewal? If so, how much of the structure of taste axons is fixed and how much remodels? Here, we measured the motility and branching of individual taste arbors (the portion of the axon innervating taste buds) in mice over time with two-photon in vivo microscopy. Terminal branches of taste arbors continuously and rapidly remodel within the taste bud. This remodeling is faster than predicted by taste bud cell renewal, with terminal branches added and lost concurrently. Surprisingly, blocking entry of new taste bud cells with chemotherapeutic agents revealed that remodeling of the terminal branches on taste arbors does not rely on the renewal of taste bud cells. Although terminal branch remodeling was fast and intrinsically controlled, no new arbors were added to taste buds, and few were lost over 100 days. Taste ganglion neurons maintain a stable number of arbors that are each capable of high-speed remodeling. We propose that terminal branch plasticity permits arbors to locate new taste bud cells while stability of arbor number supports constancy in the degree of connectivity and function for each neuron over time. Copyright:
• Sokhadze, G., Campbell, P. W., Charalambakis, N., Govindaiah, G., Guido, W., & McGee, A. W. (2022). Cre driver mouse lines for thalamocortical circuit mapping. Journal of Comparative Neurology, 530(Issue 7). doi:10.1002/cne.25248
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  Subpopulations of neurons and associated neural circuits can be targeted in mice with genetic tools in a highly selective manner for visualization and manipulation. However, there are not well-defined Cre “driver” lines that target the expression of Cre recombinase to thalamocortical (TC) neurons. Here, we characterize three Cre driver lines for the nuclei of the dorsal thalamus: Oligodendrocyte transcription factor 3 (Olig3)-Cre, histidine decarboxylase (HDC)-Cre, and corticotropin-releasing hormone (CRH)-Cre. We examined the postnatal distribution of Cre expression for each of these lines with the Cre-dependent reporter CAG-tdTomato (Ai9). Cre-dependent expression of tdTomato reveals that Olig3-Cre expresses broadly within the thalamus, including TC neurons and interneurons, while HDC-Cre and CRH-Cre each have unique patterns of expression restricted to TC neurons within and across the sensory relay nuclei of the dorsal thalamus. Cre expression is present by the time of natural birth in all three lines, underscoring their utility for developmental studies. To demonstrate the utility of these Cre drivers for studying sensory TC circuitry, we targeted the expression of channelrhodopsin-2 to thalamus from the CAG-COP4*H134R/EYFP (Ai32) allele with either HDC-Cre or CRH-Cre. Optogenetic activation of TC afferents in primary visual cortex was sufficient to measure frequency-dependent depression. Thus, these Cre drivers provide selective Cre-dependent gene expression in thalamus suitable for both anatomical and functional studies.
• Boone, H., Samonds, J., Crouse, E., Barr, C., Priebe, N., & McGee, A. (2021). Natural binocular depth discrimination behavior in mice explained by visual cortical activity. Current Biology, 31(10). doi:10.1016/j.cub.2021.02.031
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  In mice and other mammals, forebrain neurons integrate right and left eye information to generate a three-dimensional representation of the visual environment. Neurons in the visual cortex of mice are sensitive to binocular disparity,1–3 yet it is unclear whether that sensitivity is linked to the perception of depth.4–8 We developed a natural task based on the classic visual cliff and pole descent tasks to estimate the psychophysical range of mouse depth discrimination.5,9 Mice with binocular vision descended to a near (shallow) surface more often when surrounding far (deep) surfaces were progressively more distant. Occlusion of one eye severely impaired their ability to target the near surface. We quantified the distance at which animals make their decisions to estimate the binocular image displacement of the checkerboard pattern on the near and far surfaces. Then, we assayed the disparity sensitivity of large populations of binocular neurons in primary visual cortex (V1) using two-photon microscopy2 and quantitatively compared this information available in V1 to their behavioral sensitivity. Disparity information in V1 matches the behavioral performance over the range of depths examined and was resistant to changes in binocular alignment. These findings reveal that mice naturally use stereoscopic cues to guide their behavior and indicate a neural basis for this depth discrimination task.
• Frantz, M., Crouse, E., Sokhadze, G., Ikrar, T., Stephany, C., Nguyen, C., Xu, X., & McGee, A. (2020). Layer 4 Gates Plasticity in Visual Cortex Independent of a Canonical Microcircuit. Current Biology, 30(15). doi:10.1016/j.cub.2020.05.067
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  Disrupting binocular vision during a developmental critical period can yield enduring changes to ocular dominance (OD) in primary visual cortex (V1). Here we investigated how this experience-dependent plasticity is coordinated within the laminar circuitry of V1 by deleting separately in each cortical layer (L) a gene required to close the critical period, nogo-66 receptor (ngr1). Deleting ngr1 in excitatory neurons in L4, but not in L2/3, L5, or L6, prevented closure of the critical period, and adult mice remained sensitive to brief monocular deprivation. Intracortical disinhibition, but not thalamocortical disinhibition, accompanied this OD plasticity. Both juvenile wild-type mice and adult mice lacking ngr1 in L4 displayed OD plasticity that advanced more rapidly L4 than L2/3 or L5. Interestingly, blocking OD plasticity in L2/3 with the drug AM-251 did not impair OD plasticity in L5. We propose that L4 restricts disinhibition and gates OD plasticity independent of a canonical cortical microcircuit. Frantz et al. explore the regulation and propagation of experience-dependent plasticity within the laminar circuitry of visual cortex. Layer 4 limits intracortical disinhibition to close the critical period for OD plasticity throughout visual cortex. OD plasticity does not follow a canonical cortical microcircuit.
• Solomon, A., Field, G., McGee, A., Field, G. D., McGee, A. W., Solomon, A. M., & Westbrook, T. (2018). Nogo receptor 1 is expressed by nearly all retinal ganglion cells. PLoS ONE, 13(5). doi:10.1371/journal.pone.0196565
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  A variety of conditions ranging from glaucoma to blunt force trauma lead to optic nerve atrophy. Identifying signaling pathways for stimulating axon growth in the optic nerve may lead to treatments for these pathologies. Inhibiting signaling by the nogo-66 receptor 1 (NgR1) promotes the re-extension of axons following a crush injury to the optic nerve, and while NgR1 mRNA and protein expression are observed in the retinal ganglion cell (RGC) layer and inner nuclear layer, which retinal cell types express NgR1 remains unknown. Here we determine the expression pattern of NgR1 in the mouse retina by co-labeling neurons with characterized markers of specific retinal neurons together with antibodies specific for NgR1 or Green Fluorescent Protein expressed under control of the ngr1 promoter. We demonstrate that more than 99% of RGCs express NgR1. Thus, inhibiting NgR1 function may ubiquitously promote the regeneration of axons by RGCs. These results provide additional support for the therapeutic potential of NgR1 signaling in reversing optic nerve atrophy.
• Stephany, C., Ma, X., Dorton, H., Wu, J., Solomon, A., Frantz, M., Qiu, S., & McGee, A. (2018). Distinct Circuits for Recovery of Eye Dominance and Acuity in Murine Amblyopia. Current Biology, 28(12). doi:10.1016/j.cub.2018.04.055
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  Degrading vision by one eye during a developmental critical period yields enduring deficits in both eye dominance and visual acuity. A predominant model is that “reactivating” ocular dominance (OD) plasticity after the critical period is required to improve acuity in amblyopic adults. However, here we demonstrate that plasticity of eye dominance and acuity are independent and restricted by the nogo-66 receptor (ngr1) in distinct neuronal populations. Ngr1 mutant mice display greater excitatory synaptic input onto both inhibitory and excitatory neurons with restoration of normal vision. Deleting ngr1 in excitatory cortical neurons permits recovery of eye dominance but not acuity. Reciprocally, deleting ngr1 in thalamus is insufficient to rectify eye dominance but yields improvement of acuity to normal. Abolishing ngr1 expression in adult mice also promotes recovery of acuity. Together, these findings challenge the notion that mechanisms for OD plasticity contribute to the alterations in circuitry that restore acuity in amblyopia. Early visual deprivation yields enduring deficits in both eye dominance and acuity. Stephany et al. demonstrate that ngr1 limits restoration of eye dominance and acuity within distinct components of visual circuitry to reveal that recovery of these facets of vision is independent in a murine model of amblyopia.
• Frantz, M., Kast, R., Dorton, H., Chapman, K., & McGee, A. (2016). Nogo Receptor 1 Limits Ocular Dominance Plasticity but not Turnover of Axonal Boutons in a Model of Amblyopia. Cerebral Cortex, 26(5). doi:10.1093/cercor/bhv014
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  The formation and stability of dendritic spines on excitatory cortical neurons are correlated with adult visual plasticity, yet how the formation, loss, and stability of postsynaptic spines register with that of presynaptic axonal varicosities is unknown. Monocular deprivation has been demonstrated to increase the rate of formation of dendritic spines in visual cortex. However, we find that monocular deprivation does not alter the dynamics of intracortical axonal boutons in visual cortex of either adult wild-type (WT) mice or adult NgR1 mutant (ngr1-/-) mice that retain critical period visual plasticity. Restoring normal vision for a week following long-term monocular deprivation (LTMD), a model of amblyopia, partially restores ocular dominance (OD) in WT and ngr1-/- mice but does not alter the formation or stability of axonal boutons. Both WT and ngr1-/- mice displayed a rapid return of normal OD within 8 days after LTMD as measured with optical imaging of intrinsic signals. In contrast, single-unit recordings revealed that ngr1-/- exhibited greater recovery of OD by 8 days post-LTMD. Our findings support a model of structural plasticity in which changes in synaptic connectivity are largely postsynaptic. In contrast, axonal boutons appear to be stable during changes in cortical circuit function.
• Stephany, C. É., Ikrar, T., Nguyen, C., Xu, X., & McGee, A. W. (2016). Nogo receptor 1 confines a disinhibitory microcircuit to the critical period in visual cortex. Journal of Neuroscience, 36(Issue 43). doi:10.1523/jneurosci.0935-16.2016
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  A characteristic of the developing mammalian visual system is a brief interval of plasticity, termed the “critical period,” when the circuitry of primary visual cortex is most sensitive to perturbation of visual experience. Depriving one eye of vision (monocular deprivation [MD]) during the critical period alters ocular dominance (OD) by shifting the responsiveness of neurons in visual cortex to favor the nondeprived eye. A disinhibitory microcircuit involving parvalbumin-expressing (PV) interneurons initiates this OD plasticity. The gene encoding the neuronal nogo-66-receptor1(ngr1/rtn4r) is required to close the critical period. Herewecombinedmousegenetics, electrophysiology,andcircuitmapping with laser-scanning photostimulation to investigate whether disinhibition is confined to the critical period by ngr1.We demonstrate that ngr1 mutant mice retain plasticity characteristic of the critical period as adults, and that ngr1 operates within PV interneurons to restrict the loss of intracortical excitatory synaptic input following MD in adult mice, and this disinhibition induces a “lower PV network configuration” in both critical-period wild-type miceandadult ngr1-/- mice.Wepropose that ngr1 limits disinhibition to close the critical period forODplasticityand that a decrease in PV expression levels reports the diminished recent cumulative activity of these interneurons.
• Arnett, M., Herman, D., & McGee, A. (2014). Deficits in tactile learning in a mouse model of fragile x syndrome. PLoS ONE, 9(10). doi:10.1371/journal.pone.0109116
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  The fragile X mental retardation 1 mutant mouse (Fmr1 KO) recapitulates several of the neurologic deficits associated with Fragile X syndrome (FXS). As tactile hypersensitivity is a hallmark of FXS, we examined the sensory representation of individual whiskers in somatosensory barrel cortex of Fmr1 KO and wild-type (WT) mice and compared their performance in a whisker-dependent learning paradigm, the gap cross assay. Fmr1 KO mice exhibited elevated responses to stimulation of individual whiskers as measured by optical imaging of intrinsic signals. In the gap cross task, initial performance of Fmr1 KO mice was indistinguishable from WT controls. However, while WT mice improved significantly with experience at all gap distances, Fmr1 KO mice displayed significant and specific deficits in improvement at longer distances which rely solely on tactile information from whiskers. Thus, Fmr1 KO mice possess altered cortical responses to sensory input that correlates with a deficit in tactile learning.
• Park, J., Frantz, M., Kast, R., Chapman, K., Dorton, H., Stephany, C., Arnett, M., Herman, D., & McGee, A. (2014). Nogo receptor 1 limits tactile task performance independent of basal anatomical plasticity. PLoS ONE, 9(11). doi:10.1371/journal.pone.0112678
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  The genes that govern how experience refines neural circuitry and alters synaptic structural plasticity are poorly understood. The nogo-66 receptor 1 gene (ngr1) is one candidate that may restrict the rate of learning as well as basal anatomical plasticity in adult cerebral cortex. To investigate if ngr1 limits the rate of learning we tested adult ngr1 null mice on a tactile learning task. Ngr1 mutants display greater overall performance despite a normal rate of improvement on the gap-cross assay, a whisker-dependent learning paradigm. To determine if ngr1 restricts basal anatomical plasticity in the associated sensory cortex, we repeatedly imaged dendritic spines and axonal varicosities of both constitutive and conditional adult ngr1 mutant mice in somatosensory barrel cortex for two weeks through cranial windows with two-photon chronic in vivo imaging. Neither constant nor acute deletion of ngr1 affected turnover or stability of dendritic spines or axonal boutons. The improved performance on the gap-cross task is not attributable to greater motor coordination, as ngr1 mutant mice possess a mild deficit in overall performance and a normal learning rate on the rotarod, a motor task. Mice lacking ngr1 also exhibit normal induction of tone-associated fear conditioning yet accelerated fear extinction and impaired consolidation. Thus, ngr1 alters tactile and motor task performance but does not appear to limit the rate of tactile or motor learning, nor determine the low set point for synaptic turnover in sensory cortex.
• Stephany, C., Chan, L., Parivash, S., Dorton, H., Piechowicz, M., Qiu, S., & McGee, A. (2014). Plasticity of binocularity and visual acuity are differentially limited by nogo receptor. Journal of Neuroscience, 34(35). doi:10.1523/jneurosci.0545-14.2014
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  The closure of developmental critical periods consolidates neural circuitry but also limits recovery from early abnormal sensory experience. Degrading vision by one eye throughout a critical period both perturbs ocular dominance (OD) in primary visual cortex and impairs visual acuity permanently. Yet understanding how binocularity and visual acuity interrelate has proven elusive. Here we demonstrate the plasticity of binocularity and acuity are separable and differentially regulated by the neuronal nogo receptor 1 (NgR1). Mice lacking NgR1 display developmental OD plasticity as adults and their visual acuity spontaneously improves after prolonged monocular deprivation. Restricting deletion of NgR1 to either cortical interneurons or a subclass of parvalbumin (PV)-positive interneurons alters intralaminar synaptic connectivity in visual cortex and prevents closure of the critical period for OD plasticity. However, loss of NgR1 in PV neurons does not rescue deficits in acuity induced by chronic visual deprivation. Thus, NgR1 functions with PV interneurons to limit plasticity of binocularity, but its expression is required more extensively within brain circuitry to limit improvement of visual acuity following chronic deprivation. © 2014 the authors.
• Gross, G., Junge, J., Mora, R., Kwon, H., Olson, C., Takahashi, T., Liman, E., Ellis-Davies, G., McGee, A., Sabatini, B., Roberts, R., & Arnold, D. (2013). Recombinant Probes for Visualizing Endogenous Synaptic Proteins in Living Neurons. Neuron, 78(6). doi:10.1016/j.neuron.2013.04.017
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  The ability to visualize endogenous proteins in living neurons provides a powerful means to interrogate neuronal structure and function. Here we generaterecombinant antibody-like proteins, termed Fibronectin intrabodies generated with m. RNA display (FingRs), that bind endogenous neuronal proteins PSD-95 and Gephyrin with high affinity and that, when fused to GFP, allow excitatory and inhibitory synapses to be visualized in living neurons. Design of the FingR incorporates a transcriptional regulation system that ties FingR expression to the level of the target and reduces background fluorescence. In dissociated neurons and brain slices, FingRs generated against PSD-95 and Gephyrin did not affect theexpression patterns of their endogenous target proteins or the number or strength of synapses. Together, our data indicate that PSD-95 and Gephyrin FingRs can report the localization and amount of endogenous synaptic proteins in living neurons and thus may be used to study changes in synaptic strength invivo
• Chong, S., Rosenberg, S., Fancy, S., Zhao, C., Shen, Y., Hahn, A., McGee, A., Xu, X., Zheng, B., Zhang, L., Rowitch, D., Franklin, R., Lu, Q., & Chan, J. (2012). Neurite outgrowth inhibitor Nogo-A establishes spatial segregation and extent of oligodendrocyte myelination. Proceedings of the National Academy of Sciences of the United States of America, 109(4). doi:10.1073/pnas.1113540109
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  A requisite component of nervous system development is the achievement of cellular recognition and spatial segregation through competition-based refinement mechanisms. Competition for available axon space by myelinating oligodendrocytes ensures that all relevant CNS axons are myelinated properly. To ascertain the nature of this competition, we generated a transgenic mouse with sparsely labeled oligodendrocytes and establish that individual oligodendrocytes occupying similar axon tracts can greatly vary the number and lengths of their myelin internodes. Here we show that intercellular interactions between competing oligodendroglia influence the number and length of myelin internodes, referred to as myelinogenic potential, and identify the amino-terminal region of Nogo-A, expressed by oligodendroglia, as necessary and sufficient to inhibit this process. Exuberant and expansive myelination/remyelination is detected in the absence of Nogo during development and after demyelination, suggesting that spatial segregation and myelin extent is limited by microenvironmental inhibition. We demonstrate a unique physiological role for Nogo-A in the precise myelination of the developing CNS. Maximizing the myelinogenic potential of oligodendrocytes may offer an effective strategy for repair in future therapies for demyelination.
• Wang, X., Duffy, P., McGee, A., Hasan, O., Gould, G., Tu, N., Harel, N., Huang, Y., Carson, R., Weinzimmer, D., Ropchan, J., Benowitz, L., Cafferty, W., & Strittmatter, S. (2011). Recovery from chronic spinal cord contusion after nogo receptor intervention. Annals of Neurology, 70(5). doi:10.1002/ana.22527
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  Objective: Several interventions promote axonal growth and functional recovery when initiated shortly after central nervous system injury, including blockade of myelin-derived inhibitors with soluble Nogo receptor (NgR1, RTN4R) decoy protein. We examined the efficacy of this intervention in the much more prevalent and refractory condition of chronic spinal cord injury. Methods: We eliminated the NgR1 pathway genetically in mice by conditional gene targeting starting 8 weeks after spinal hemisection injury and monitored locomotion in the open field and by video kinematics over the ensuing 4 months. In a separate pharmacological experiment, intrathecal NgR1 decoy protein administration was initiated 3 months after spinal cord contusion injury. Locomotion and raphespinal axon growth were assessed during 3 months of treatment between 4 and 6 months after contusion injury. Results: Conditional deletion of NgR1 in the chronic state results in gradual improvement of motor function accompanied by increased density of raphespinal axons in the caudal spinal cord. In chronic rat spinal contusion, NgR1 decoy treatment from 4 to 6 months after injury results in 29% (10 of 35) of rats recovering weight-bearing status compared to 0% (0 of 29) of control rats (p < 0.05). Open field Basso, Beattie, and Bresnahan locomotor scores showed a significant improvement in the NgR-treated group relative to the control group (p < 0.005, repeated measures analysis of variance). An increase in raphespinal axon density caudal to the injury is detected in NgR1 decoy-treated animals by immunohistology and by positron emission tomography using a serotonin reuptake ligand. Interpretation: Antagonizing myelin-derived inhibitors signaling with NgR1 decoy augments recovery from chronic spinal cord injury. ANN NEUROL 2011 Copyright © 2011 American Neurological Association.
• Budel, S., Padukkavidana, T., Liu, B., Feng, Z., Hu, F., Johnson, S., Lauren, J., Park, J., McGee, A., Liao, J., Stillman, A., Kim, J., Yang, B., Sodi, S., Gelernter, J., Zhao, H., Hisama, F., Arnsten, A., & Strittmatter, S. (2008). Genetic variants of Nogo-66 receptor with possible association to schizophrenia block myelin inhibition of axon growth. Journal of Neuroscience, 28(49). doi:10.1523/jneurosci.3828-08.2008
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  In schizophrenia, genetic predisposition has been linked to chromosome 22q11 and myelin-specific genes are misexpressed in schizophrenia. Nogo-66 receptor 1 (NGR or RTN4R) has been considered to be a 22q11 candidate gene for schizophrenia susceptibility because it encodes an axonal protein that mediates myelin inhibition of axonal sprouting. Confirming previous studies, we found that variation at the NGR locus is associated with schizophrenia in a Caucasian case-control analysis, and this association is not attributed to population stratification. Within a limited set of schizophrenia-derived DNA samples, we identified several rare NGR nonconservative coding sequence variants. Neuronal cultures demonstrate that four different schizophrenia-derived NgR1 variants fail to transduce myelin signals into axon inhibition, and function as dominant negatives to disrupt endogenous NgR1. This provides the first evidence that certain disease-derived human NgR1 variants are dysfunctional proteins in vitro. Mice lacking NgR1 protein exhibit reduced working memory function, consistent with a potential endophenotype of schizophrenia. For a restricted subset of individuals diagnosed with schizophrenia, the expression of dysfunctional NGR variants may contribute to increased disease risk. Copyright © 2008 Society for Neuroscience.
• Cafferty, W., McGee, A., & Strittmatter, S. (2008). Axonal growth therapeutics: regeneration or sprouting or plasticity?. Trends in Neurosciences, 31(5). doi:10.1016/j.tins.2008.02.004
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  Loss of function after neurological injury frequently occurs through the interruption of axonal connectivity, rather than through cell loss. Functional deficits persist because a multitude of inhibitory factors in degenerating myelin and astroglial scar prevent axonal growth in the adult brain and spinal cord. Given the high clinical significance of achieving functional recovery through axonal growth, substantial research effort has been, and will be, devoted toward this desirable goal. Unfortunately, the labels commonly used in the literature to categorize post-injury axonal anatomy might hinder advancement. In this article, we present an argument for the importance of developing precise terms that describe axonal growth in terms of the inciting event, the distance of axonal extension and the timing of axonal growth. The phenotypes produced by molecular interventions that overcome astroglial scar or myelin-associated inhibitors are reframed and discussed in this context. © 2008 Elsevier Ltd. All rights reserved.
• McGee, A., Yang, Y., Fischer, Q., Daw, N., & Strittmatter, S. (2005). Neuroscience: Experience-driven plasticity of visual cortex limited by myelin and nogo receptor. Science, 309(5744). doi:10.1126/science.1114362
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  Monocular deprivation normally alters ocular dominance Fn the visual cortex only during a postnatal critical period (20 to 32 days postnatal in mice). We find that mutations in the Nogo-66 receptor (NgR) affect cessation of ocular dominance plasticity. In NgR-/- mice, plasticity during the critical period is normal, but it continues abnormally such that ocular dominance at 45 or 120 days postnatal is subject to the same plasticity as at juvenile ages. Thus, physiological NgR signaling from myelin-derived Nogo, MAG, and OMgp consolidates the neural circuitry established during experience-dependent plasticity. After pathological trauma, similar NgR signaling limits functional recovery and axonal regeneration.
• McGee, A., Nunziato, D., Maltez, J., Prehoda, K., Pitt, G., & Bredt, D. (2004). Calcium channel function regulated by the SH3-GK module in β subunits. Neuron, 42(1). doi:10.1016/s0896-6273(04)00149-7
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  β subunits of voltage-gated calcium channels (VGCCs) regulate channel trafficking and function, thereby shaping the intensity and duration of intracellular changes in calcium. β subunits share limited sequence homology with the Src homology 3-guanylate kinase (SH3-GK) module of membrane-associated guanylate kinases (MAGUKs). Here, we show biochemical similarities between β subunits and MAGUKs, revealing important aspects of β subunit structure and function. Similar to MAGUKs, an SH3-GK interaction within β subunits can occur both intramolecularly and intermolecularly. Mutations that disrupt the SH3-GK interaction in β subunits alter channel inactivation and can inhibit binding between the α1 and β subunits. Coexpression of β subunits with complementary mutations in their SH3 and GK domains rescues these deficits through intermolecular β subunit assembly. In MAGUKs, the SH3-GK module controls protein scaffolding. In β subunits, this module regulates the inactivation of VGCCs and provides an additional mechanism for tuning calcium responsiveness.
• Weiss, J., Takizawa, B., McGee, A., Stewart, W., Zhang, H., Ment, L., Schwartz, M., & Strittmatter, S. (2004). Neonatal hypoxia suppresses oligodendrocyte Nogo-A and increases axonal sprouting in a rodent model for human prematurity. Experimental Neurology, 189(1). doi:10.1016/j.expneurol.2004.05.018
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  Premature human infants frequently suffer from periventricular leukomalacia (PVL) characterized by the loss of central myelinated tracts in the brain [Neuropathology, 22 (2002) 193]. Rodent chronic sublethal hypoxia (CSH) from P3 to 33 (postnatal day 3-33) provides a model for PVL characterized by cerebral ventriculomegaly and reductions in cerebral white matter volume [Brain Res. Dev. Brain Res. 111 (1998) 197; Proc. Natl. Acad. Sci. USA 100 (2003) 11718]. Here, we demonstrate that mice exposed to CSH from P3 to P33 followed by normoxia from P33 to P75 continue to exhibit a locomotor hyperactivity that resembles behavioral changes observed in some human children with very low birth weights. Because periventricular white matter is specifically lost in PVL, we examined the expression of oligodendrocyte proteins. Hypoxic rearing dramatically decreases the level of the axon outgrowth inhibitor Nogo-A in oligodendrocytes of CNS white matter at P12. The Nogo-A decrease exceeds the moderate decrease in another myelin protein, myelin associated glycoprotein (MAG). Although myelin protein expression returns to normal by maturity (P75), persistent abnormalities in axonal trajectories are detectable. Anterograde axonal tracing from motor cortex demonstrates ectopic corticofugal fibers in the corticospinal tract (CST), corpus callosum, and caudate nucleus of adult animals reared in CSH. Thus, hypoxia-induced reduction in myelin-derived axon outgrowth inhibitors appears to contribute axonal misconnection to the pathology of very low birth weight infants. © 2004 Elsevier Inc. All rights reserved.
• McGee, A., Dakoji, S., Olsen, O., Bredt, D., Lim, W., & Prehoda, K. (2001). Structure of the SH3-guanylate kinase module from PSD-95 suggests a mechanism for regulated assembly of MAGUK scaffolding proteins. Molecular Cell, 8(6). doi:10.1016/s1097-2765(01)00411-7
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  Membrane-associated guanylate kinases (MAGUKs), such as PSD-95, are modular scaffolds that organize signaling complexes at synapses and other cell junctions. MAGUKs contain PDZ domains, which recruit signaling proteins, as well as a Src homology 3 (SH3) and a guanylate kinase-like (GK) domain, implicated in scaffold oligomerization. The crystal structure of the SH3-GK module from PSD-95 reveals that these domains form an integrated unit: the SH3 fold comprises noncontiguous sequence elements divided by a hinge region and the GK domain. These elements compose two subdomains that can assemble in either an intra- or intermolecular fashion to complete the SH3 fold. We propose a model for MAGUK oligomerization in which complementary SH3 subdomains associate by 3D domain swapping. This model provides a possible mechanism for ligand regulation of oligomerization.
• McGee, A., Topinka, J., Hashimoto, K., Petralia, R., Kakizawa, S., Kauer, F., Aguilera-Moreno, A., Wenthold, R., Kano, M., & Bredt, D. (2001). PSD-93 knock-out mice reveal that neuronal MAGUKs are not required for development or function of parallel fiber synapses in cerebellum. Journal of Neuroscience, 21(9). doi:10.1523/jneurosci.21-09-03085.2001
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  Membrane-associated guanylate kinases (MAGUKs) are abundant postsynaptic density (PSD)-95/discs large/zona occludens-1 (PDZ)-containing proteins that can assemble receptors and associated signaling enzymes at sites of cell-cell contact, including synapses. PSD-93, a postsynaptic neuronal MAGUK, has three PDZ domains that can bind to specific ion channels, including NMDA δ2 type glutamate receptors, as well as Shaker and inward rectifier type K+ channels, and can mediate clustering of these channels in heterologous cells. Genetic analyses of Drosophila show that MAGUKs play critical roles in synaptic development because mutations of discs large disrupt the subsynaptic reticulum and block postsynaptic clustering of Shaker K+ channels. It is uncertain whether MAGUKs play an essential role in the development of central synapses. There are four neuronal MAGUKs with overlapping expression patterns in the mammalian brain; however, we find PSD-93 is the only MAGUK expressed in cerebellar Purkinje neurons. Therefore, we targeted disruption of PSD-93 in mouse. Despite the absence of MAGUK immunoreactivity in Purkinje neurons from the knock-outs, these mice have no structural or functional abnormality in cerebellum. Both the dendritic architecture and the postsynaptic localization of PSD-93 interacting proteins remain intact at light and electron microscopic levels in the knock-outs. Postsynaptic Purkinje cell responses, monosynaptic climbing fiber innervation, and cerebellar-dependent behaviors are also normal. Our data demonstrate that MAGUK proteins of the PSD-93/95 family are not essential for development of certain central synapses but may instead participate in specialized aspects of synaptic signaling and plasticity.
• McGee, A., & Bredt, D. (1999). Identification of an intramolecular interaction between the SH3 and guanylate kinase domains of PSD-95. Journal of Biological Chemistry, 274(25). doi:10.1074/jbc.274.25.17431
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  Postsynaptic density-95 (PSD-95/SAP-90) is a member of the membrane- associated guanylate kinase (MAGUK) family of proteins that assemble protein complexes at synapses and other cell junctions. MAGUKs comprise multiple protein-protein interaction motifs including PDZ, SH3 and guanylate kinase (GK) domains, and these binding sites mediate the scaffolding function of MAGUK proteins. Synaptic binding partners for the PDZ and GK domains of PSD- 95 have been identified, but the role of the SH3 domain remains elusive. We now report that the SH3 domain of PSD-95 mediates a specific interaction with the GK domain. The GK domain lacks a poly-proline motif that typically binds to SH3 domains; instead, SH3/GK binding is a bi-domain interaction that requires both intact motifs. Although isolated SH3 and GK domains can bind in trans, experiments with intact PSD-95 molecules indicate that intramolecular SH3/GK binding dominates and prevents intermolecular associations. SH3/GK binding is conserved in the related Drosophila MAGUK protein DLG but is not detectable for Caenorhabditis elegans LIN-2. Many previously identified genetic mutations of MAGUKs in invertebrates occur in the SH3 or GK domains, and all of these mutations disrupt intramolecular SH3/GK binding.
• Roche, K., Ly, C., Petralia, R., Wang, Y., McGee, A., Bredt, D., & Wenthold, R. (1999). Postsynaptic density-93 interacts with the δ2 glutamate receptor subunit at parallel fiber synapses. Journal of Neuroscience, 19(10). doi:10.1523/jneurosci.19-10-03926.1999
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  The glutamate receptor subunit δ2 has a unique distribution at the parallel fiber-Purkinje cell synapse of the cerebellum, which is developmentally regulated such that δ2 occurs at both parallel fiber synapses and climbing fiber synapses early in development but is restricted to parallel fiber synapses in adult animals. To identify proteins that might be involved in the trafficking or docking of δ2 receptors, we screened a yeast two-hybrid library with the cytosolic C terminus of δ2 and isolated a member of the postsynaptic density (PSD)-95 family of proteins, which are known to interact with the extreme C termini of NMDA receptors. We find that δ2 binds specifically to PSD-93, which is enriched in Purkinje cells. In addition, PSD-93 clusters δ2 when they are coexpressed in heterologous cells, and clustering is disrupted by point mutations of δ2 that disrupt the δ2-PSD-93 interaction. Ultrastructural localization of PSD-93 and δ2 shows they are colocalized at parallel fiber synapses; however, PSD-93 also is present at climbing fiber synapses of the adult rat, where δ2 is not found, indicating that the presence of PSD-93 alone is not sufficient for determining the synaptic expression of δ2.
• Brenman, J., Rick Topinka, J., Cooper, E., McGee, A., Rosen, J., Milroy, T., Ralston, H., & Bredt, D. (1998). Localization of postsynaptic density-93 to dendritic microtubules and interaction with microtubule-associated protein 1A. Journal of Neuroscience, 18(21). doi:10.1523/jneurosci.18-21-08805.1998
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  Postsynaptic density-93 (PSD-93)/Chapsyn-110 is a member of the membrane-associated guanylate kinase (MAGUK) family of PDZ domain-containing proteins. MAGUKs are widely expressed in the brain and are critical elements of the cytoskeleton and of certain synapses. In the ultrastructural studies that are described here, PSD-93 localizes to both postsynaptic densities and dendritic microtubules of cerebellar Purkinje neurons. The microtubule localization is paralleled by a high-affinity in vivo interaction of PSD-93 via its guanylate kinase (GK) domain with microtubule-associated protein 1A (MAP1A). GK domain truncations that mimic genetically identified mutations of a Drosophila MAGUK, discs-large, disrupt the GK/MAP-1A interaction. Additional biochemical experiments demonstrate that intact MAGUKs do not bind to MAP1A as effectively as do isolated GK domains. This appears to be attributable to an intramolecular inhibition of the GK domain by the PDZs, because GK binding activity of full-length MAGUKs is partially restored by a variety of PDZ ligands, including the C termini of NMDA receptor 2B, adenomatous polyposis coli (APC), and CRIPT. Beyond demonstrating a novel cytoskeletal link for PSD-93, these experiments support a model in which intramolecular interactions between the multiple domains of MAGUKs regulate intermolecular associations and thereby may play a role in the proper targeting and function of MAGUK proteins.
• Brenman, J., Chao, D., Gee, S., McGee, A., Craven, S., Santillano, D., Wu, Z., Huang, F., Xia, H., Peters, M., Froehner, S., & Bredt, D. (1996). Interaction of nitric oxide synthase with the postsynaptic density protein PSD-95 and α1-syntrophin mediated by PDZ domains. Cell, 84(5). doi:10.1016/s0092-8674(00)81053-3
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  Neuronal nitric oxide synthase (nNOS) is concentrated at synaptic junctions in brain and motor endplates in skeletal muscle. Here, we show that the N-terminus of nNOS, which contains a PDZ protein motif, interacts with similar motifs in postsynaptic density-95 protein (PSD-95) and a related novel protein, PSD-93. nNOS and PSD-95 are coexpressed in numerous neuronal populations, and a PSD-95/nNOS complex occurs in cerebellum. PDZ domain interactions also mediate binding of nNOS to skeletal muscle syntrophin, a dystrophin-associated protein. nNOS isoforms lacking a PDZ domain, identified in nNOS(Δ/Δ) mutant mice, do not associate with PSD-95 in brain or with skeletal muscle sarcolemma. Interaction of PDZ-containing domains therefore mediates synaptic association of nNOS and may play a more general role in formation of macromolecular signaling complexes.
• Brenman, J., Christopherson, K., Craven, S., McGee, A., & Bredt, D. (1996). Cloning and characterization of postsynaptic density 93, a nitric oxide synthase interacting protein. Journal of Neuroscience, 16(23). doi:10.1523/jneurosci.16-23-07407.1996
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  Nitric oxide (NO) formation in brain is regulated by the calcium/calmodulin dependence of neuronal NO synthase (nNOS). Calcium influx through NMDA-type glutamate receptors is efficiently coupled to nNOS activity, whereas many other intracellular calcium pathways are poorly coupled. To elucidate possible mechanisms responsible for this coupling, we performed yeast two-hybrid screening to identify proteins that interact with nNOS. Two nNOS interacting proteins were identified: the postsynaptic density proteins PSD-93 and PSD-95. Here, we report the cloning and characterization of PSD-93. PSD-93 is expressed in discrete neuronal populations as well as in specific non-neuronal cells, and it exhibits complex molecular diversity attributable to tissue-specific alternative splicing. PSD-93, like PSD-95, binds to nNOS and to the NMDA receptor 2B. PSD-93, however, is unique among PSD-95/SAP-90 family members in its expression in Purkinje neuron cell bodies and dendrites. We also demonstrate that the PDZ domain at the N terminus of nNOS is required, but it is not sufficient for interaction with PSD-93/95. Given that PSD-93 and PSD-95 each contain multiple potential binding sites for nNOS and the NMDA receptor, complexes involving oligomers of PSD-93/95 may help account for the functional as well as the physical coupling of nNOS to NMDA receptors.