Recovery after central nervous system (CNS) injury has long been a challenge for clinical investigators. Blockade of the oligodendrocyte-associated inhibitor Nogo-A has shown great promise in promoting neuronal regeneration, sprouting, and plasticity, as well as functional recovery in rodent and primate models of CNS injury. The high expression of Nogo-A in neurons of the postnatal CNS led us to look for potential roles of this protein in this stage of development. We hypothesized that postnatal, neuronal NogoA influences the density and morphology of dendritic spines in the developing CNS, in part, by regulating the maturation and stability of glutamatergic synaptic input.

To examine the roles of Nogo-A at the excitatory synapse of the neocortex, we used RNAi directed against Nogo-A and delivered it to the developing rat sensorimotor cortex via AAV2/8, a neurotropic vector. This resulted in lowered density of dendritic spines, which are known to house over 90% of excitatory connections onto pyramidal neurons. This decrease was particularly evident with thin- and mushroom-shaped spines in dendrites of the apical arbor. A decrease in protrusions with moderately-wide head widths and very long necks was also noted. We then used vesicular glutamate transporter 1 (vGlut1) as a marker for potential excitatory synapses. Knocking down Nogo-A in postnatal pyramidal neurons of the sensorimotor cortex led to a decrease in the number of vGlut1 puncta identifying a potential presynaptic partner, in opposition to the apical dendritic shaft. The decreased vGlut1 in the apical arbor likely represents a loss of potential synapses that may have a strong influence on direct current injected into the dendrite.

We further examined regions of transduced cortex via qRT-PCR for message levels of molecules important for plasticity at the excitatory synapse, including Neuroligin-1, NMDA receptor subunits NR2A and NR2B, and PSD-95. We found that mRNA of Neuroligin-1 and NR2B was substantially reduced, with no changes to NR2A or PSD-95 expression. These results suggest that neuronal Nogo-A may act to maintain elements of the neocortical excitatory synapse during development. This finding represents a novel role for Nogo-A in the intact CNS.