Cortical plasticity: structural and functional changes in cortical neurons and their connections

The cerebral cortex is one of the most complex structures, with an estimated 100 billion neurons connected with one another. One of the key features of the cortex that allows us to adapt our behavior in response to experience is its plasticity, i.e. the ability to reorganize and rewire structural and functional connections in response to changes in the environment. A great deal of progress has been made toward understanding cortical plasticity. This dissertation focuses on how distinct sensory stimuli induce structural and functional plasticity. First, using in vivo two photon imaging, we investigate the effect of two types of experience-dependent plasticity, motor learning and sensory deprivation, on structural plasticity of distinct cortical layer neurons. Our data reveal that neurons in different cortical layers exhibit distinct structural plasticity of apical dendritic spines, which may arise from their distinct functional roles in cortical circuits. Second, we focus on molecular mechanisms of spine and cortical circuit structural plasticity. Here we show that retinoic acid (RA) signaling plays an essential role in dendritic spine experience-dependent plasticity in vivo. Finally, combining in vivo two photon calcium imaging with behavioral analysis, we investigate the effect of stress on neuronal responses and functional responses of layer 2/3 neurons, which play an important role in sensory processing. The proposed studies will provide circuit-level insight into how the cortex responds to changes in the environment

Differential roles for EphA and EphB signaling in segregation and patterning of central vestibulocochlear nerve projections.

Auditory and vestibular afferents enter the brainstem through the VIIIth cranial nerve and find targets in distinct brain regions. We previously reported that the axon guidance molecules EphA4 and EphB2 have largely complementary expression patterns in the developing avian VIIIth nerve. Here, we tested whether inhibition of Eph signaling alters central targeting of VIIIth nerve axons. We first identified the central compartments through which auditory and vestibular axons travel. We then manipulated Eph-ephrin signaling using pharmacological inhibition of Eph receptors and in ovo electroporation to misexpress EphA4 and EphB2. Anterograde labeling of auditory afferents showed that inhibition of Eph signaling did not misroute axons to non-auditory target regions. Similarly, we did not find vestibular axons within auditory projection regions. However, we found that pharmacologic inhibition of Eph receptors reduced the volume of the vestibular projection compartment. Inhibition of EphB signaling alone did not affect auditory or vestibular central projection volumes, but it significantly increased the area of the auditory sensory epithelium. Misexpression of EphA4 and EphB2 in VIIIth nerve axons resulted in a significant shift of dorsoventral spacing between the axon tracts, suggesting a cell-autonomous role for the partitioning of projection areas along this axis. Cochlear ganglion volumes did not differ among treatment groups, indicating the changes seen were not due to a gain or loss of cochlear ganglion cells. These results suggest that Eph-ephrin signaling does not specify auditory versus vestibular targets but rather contributes to formation of boundaries for patterning of inner ear projections in the hindbrain

Postnatal Ablation of Synaptic Retinoic Acid Signaling Impairs Cortical Information Processing and Sensory Discrimination in Mice

Retinoic acid (RA) and its receptors (RARs) are well established essential transcriptional regulators during embryonic development. Recent findings in cultured neurons identified an independent and critical post-transcriptional role of RA and RARα in the homeostatic regulation of excitatory and inhibitory synaptic transmission in mature neurons. However, the functional relevance of synaptic RA signaling in vivo has not been established. Here, using somatosensory cortex as a model system and the RARα conditional knock-out mouse as a tool, we applied multiple genetic manipulations to delete RARα postnatally in specific populations of cortical neurons, and asked whether synaptic RA signaling observed in cultured neurons is involved in cortical information processing in vivo Indeed, conditional ablation of RARα in mice via a CaMKIIα-Cre or a layer 5-Cre driver line or via somatosensory cortex-specific viral expression of Cre-recombinase impaired whisker-dependent texture discrimination, suggesting a critical requirement of RARα expression in L5 pyramidal neurons of somatosensory cortex for normal tactile sensory processing. Transcranial two-photon imaging revealed a significant increase in dendritic spine elimination on apical dendrites of somatosensory cortical layer 5 pyramidal neurons in these mice. Interestingly, the enhancement of spine elimination is whisker experience-dependent as whisker trimming rescued the spine elimination phenotype. Additionally, experiencing an enriched environment improved texture discrimination in RARα-deficient mice and reduced excessive spine pruning. Thus, RA signaling is essential for normal experience-dependent cortical circuit remodeling and sensory processing.SIGNIFICANCE STATEMENT The importance of synaptic RA signaling has been demonstrated in in vitro studies. However, whether RA signaling mediated by RARα contributes to neural circuit functions in vivo remains largely unknown. In this study, using a RARα conditional knock-out mouse, we performed multiple regional/cell-type-specific manipulation of RARα expression in the postnatal brain, and show that RARα signaling contributes to normal whisker-dependent texture discrimination as well as regulating spine dynamics of apical dendrites from layer (L5) pyramidal neurons in S1. Deletion of RARα in excitatory neurons in the forebrain induces elevated spine elimination and impaired sensory discrimination. Our study provides novel insights into the role of RARα signaling in cortical processing and experience-dependent spine maturation

Postnatal Ablation of Synaptic Retinoic Acid Signaling Impairs Cortical Information Processing and Sensory Discrimination in Mice

Retinoic acid (RA) and its receptors (RARs) are well established essential transcriptional regulators during embryonic development. Recent findings in cultured neurons identified an independent and critical post-transcriptional role of RA and RARα in the homeostatic regulation of excitatory and inhibitory synaptic transmission in mature neurons. However, the functional relevance of synaptic RA signaling in vivo has not been established. Here, using somatosensory cortex as a model system and the RARα conditional knock-out mouse as a tool, we applied multiple genetic manipulations to delete RARα postnatally in specific populations of cortical neurons, and asked whether synaptic RA signaling observed in cultured neurons is involved in cortical information processing in vivo Indeed, conditional ablation of RARα in mice via a CaMKIIα-Cre or a layer 5-Cre driver line or via somatosensory cortex-specific viral expression of Cre-recombinase impaired whisker-dependent texture discrimination, suggesting a critical requirement of RARα expression in L5 pyramidal neurons of somatosensory cortex for normal tactile sensory processing. Transcranial two-photon imaging revealed a significant increase in dendritic spine elimination on apical dendrites of somatosensory cortical layer 5 pyramidal neurons in these mice. Interestingly, the enhancement of spine elimination is whisker experience-dependent as whisker trimming rescued the spine elimination phenotype. Additionally, experiencing an enriched environment improved texture discrimination in RARα-deficient mice and reduced excessive spine pruning. Thus, RA signaling is essential for normal experience-dependent cortical circuit remodeling and sensory processing.SIGNIFICANCE STATEMENT The importance of synaptic RA signaling has been demonstrated in in vitro studies. However, whether RA signaling mediated by RARα contributes to neural circuit functions in vivo remains largely unknown. In this study, using a RARα conditional knock-out mouse, we performed multiple regional/cell-type-specific manipulation of RARα expression in the postnatal brain, and show that RARα signaling contributes to normal whisker-dependent texture discrimination as well as regulating spine dynamics of apical dendrites from layer (L5) pyramidal neurons in S1. Deletion of RARα in excitatory neurons in the forebrain induces elevated spine elimination and impaired sensory discrimination. Our study provides novel insights into the role of RARα signaling in cortical processing and experience-dependent spine maturation

Characterization of auditory and vestibular compartments in the E8 hindbrain.

<p>(<b>A</b>) Low power image of coronal section immunolabeled for neurofilament (NF). The region containing the VIIIth nerve central projections is shown in a rectangle in upper right. Dorsal is up in all all figures. (<b>B</b>) Schematic diagram showing the key features in the boxed region in A. Neurofilament reveals the ipsilateral n. magnocellularis projection (iNM) and the contralateral projection (cNM), seen ventral to n. laminaris (NL). (<b>C</b>-<b>E</b>) Selective labeling of high and low frequency regions of cochlear ganglion cell fibers with RDA reveals the auditory extent of the hindbrain at the level of VIIIth nerve entry. The traced high frequency axons (arrow) are dorsal to the traced lower frequency axons and can be visualized in the hindbrain across several coronal sections (labeled rostral, middle, caudal). (<b>F</b>) Co-staining with neurofilament antibody demonstrates an example in which GFP transfected axons are found only in vestibular ganglion cells. NM-NL fiber tract (cNM fibers, outlined in A-C) acts as a landmark, and is used to demarcate between auditory and vestibular compartments (arrowheads). (<b>G</b>) Example in which both auditory and vestibular ganglion cells are transfected. (<b>H</b>) Embryo with both auditory and vestibular ganglion axons transfected with GFP and subjected to RDA tracing. Central RDA label shows the location of a subset of auditory fibers, which falls within characterized auditory compartment. Scale bars in A, 500 µm; in C (applies to D,E), F, G, and H, 100 µm.</p