Nogo-66 Receptor at Cerebellar Cortical Glia Gap Junctions in the Rat

Astrocytes as a target for Nogo-A and implications for synapse formation in vitro and in a model of acute demyelination

Central nervous system (CNS) function depends on precise synaptogenesis, which is shaped by environmental cues and cellular interactions. Astrocytes are outstanding regulators of synapse development and plasticity through contact-dependent signals and through the release of pro- and antisynaptogenic factors. Conversely, myelin and its associated proteins, including Nogo-A, affect synapses in a inhibitory fashion and contribute to neural circuitry stabilization. However, the roles of Nogo-A-astrocyte interactions and their implications in synapse development and plasticity have not been characterized. Therefore, we aimed to investigate whether Nogo-A affects the capacity of astrocytes to induce synaptogenesis. Additionally, we assessed whether downregulation of Nogo-A signaling in an in vivo demyelination model impacts the synaptogenic potential of astrocytes. Our in vitro data show that cortical astrocytes respond to Nogo-A through RhoA pathway activation, exhibiting stress fiber formation and decreased ramified morphology. This phenotype was associated with reduced levels of GLAST protein and aspartate uptake, decreased mRNA levels of the synaptogenesis-associated genes Hevin, glypican-4, TGF-β1 and BDNF, and decreased and increased protein levels of Hevin and SPARC, respectively. Corroborating these findings, conditioned medium from Nogo-A-treated astrocytes suppressed the formation of structurally and functionally mature synapses in cortical neuronal cultures. After cuprizone-induced acute demyelination, we observed reduced immunostaining for Nogo-A in the visual cortex accompanied by higher levels of Hevin expression in astrocytes and an increase in excitatory synapse density. Hence, we suggest that interactions between Nogo-A and astrocytes might represent an important pathway of plasticity regulation and could be a target for therapeutic intervention in demyelinating diseases in the future.

Formation of the Cortical Subventricular Zone Requires MDGA1-Mediated Aggregation of Basal Progenitors

The subventricular zone (SVZ) provides a specialized neurogenic microenvironment for proliferation and aggregation of basal progenitors (BPs). Our study reveals a mechanism for the aggregation of BPs within the SVZ required for their proliferation and generation of cortical layer neurons. The autism-related IgCAM, MDGA1, is locally expressed in the BP cell membrane where it co-localizes and complexes with the gap junction protein Connexin43. To address MDGA1 function, we created a floxed allele of MDGA1 and deleted it from BPs. MDGA1 deletion results in reduced BP proliferation and size of the SVZ, with an aberrant population of BPs ectopically positioned in the cortical plate. These defects are manifested in diminished production of cortical layer neurons and a significant reduction of the cortical layers. We conclude that MDGA1 functions to aggregate and maintain BPs within the SVZ providing the neurogenic niche required for their proliferation and generation of cortical layer neurons.

Nogo-66 inhibits the dye-coupling of astrocytic gap junctions in vitro

Communication between astrocytes via the gap junction is crucial for maintaining homeostasis of the extra-neuronal microenvironment of the central nervous system. Dysfunction of astrocytic gap junctions is involved in many brain disorders. Our previous studies demonstrated a novel co-localization of Nogo-66 receptor at glial gap junctions in rat cerebellum and posterior pituitary. The present study was aimed at exploring whether Nogo-66 can modulate glial gap junctions in vitro. We confirmed the co-localization of Nogo-66 receptor with Cx43 in cultured astrocytes, and stimulated astrocytes with myelin extracts, or Nogo-66-Fc conditioned medium. Finally, we expressed and purified a functionally effective GST-Nogo-66 peptide. Lucifer yellow transfer assay was adopted to measure the gap junction permeability. The results showed that the spreading of Lucifer yellow was inhibited significantly by all three treatments as compared with their corresponding controls. Therefore, this study shows a novel inhibitory effect of Nogo-66 on the permeability of astrocytic gap junctions, suggesting a presumable role of Nogo-66 receptor in modulating the glial gap junction.

Spatio-temporal expression pattern of receptors for myelin-associated inhibitors in the developing rat olfactory system

The myelin-associated inhibitory proteins (Nogo-A, MAG and OMgp) that prevent axon regeneration in adult CNS, mediate their effects via a receptor referred as NgR1. Beside their inhibitory role in the adult CNS, Nogo-A and NgR1 might also be functionally involved in the developing nervous system. At the present time, no detailed study is available regarding either the onset of NgR1 expression during development or its spatio-temporal pattern of expression relative to the presence of Nogo-A. Two homologs of NgR1, NgR2 and NgR3, have been recently identified, but their function in the nervous system is still unknown in adult as well as during development. We have examined the spatio-temporal expression pattern of both NgR1, NgR2 and NgR3 mRNAs and corresponding proteins in the developing rat olfactory system using in situ hybridization and immunohistochemistry. From E15-E16 onwards, NgR1 mRNA was expressed by differentiating neurons in both the olfactory epithelium and the olfactory bulb. At all developmental stages, including adult animals, NgR1 protein was preferentially targeted to olfactory axons emerging from the olfactory epithelium. Using double-immunostainings in the postnatal olfactory mucosa, we confirm the neuronal localization of NgR1 and its preferential distribution along the olfactory axons. The NgR2 and NgR3 transcripts and their proteins display similar expression profiles in the olfactory system. Together, our data suggest that, in non-pathological conditions, NgR1 and its homologs may play a role in axon outgrowth in the rat olfactory system and may be relevant for the confinement of neural projections within the developing olfactory bulb.

Nogo-66 receptor at the gap junctions between pituicytes of the rat

Nogo-66 receptor was first identified in neurons. Recently, it was demonstrated in glial cells as well. Our previous study on the expression of Nogo-66 receptor in the cerebellum of the rat surprisingly found its location at the glial gap junctions. Here, we present our study on Nogo-66 receptor in the rat posterior pituitary, which is densely packed with pituicytes, a special type of astrocyte, and is known to be rich in gap junctions. We were able to demonstrate with immunohistochemistry and immuno-electron microscopy abundant Nogo-66 receptor immunoreactive gap junctions between pituicytes. This study, together with our prior one, strongly suggests that the Nogo-66 receptor may play a role in regulating the function of the gap junctions.