Formation and maintenance of a neuronal network is based on a balance between plasticity and stability of synaptic connections. Several molecules have been found to regulate the maintenance of excitatory synapses but nothing is known about the molecular mechanisms involved in synaptic stabilization versus disassembly at inhibitory synapses. Here, we demonstrate that Nogo-A, which is well known to be present in myelin and inhibit growth in the adult CNS, is present in inhibitory presynaptic terminals in cerebellar Purkinje cells at the time of Purkinje cell-Deep Cerebellar Nuclei (DCN) inhibitory synapse formation and is then downregulated during synapse maturation. We addressed the role of neuronal Nogo-A in synapse maturation by generating several mouse lines overexpressing Nogo-A, starting at postnatal ages and throughout adult life, specifically in cerebellar Purkinje cells and their terminals. The overexpression of Nogo-A induced a progressive disassembly, retraction and loss of the inhibitory Purkinje cell terminals. This led to deficits in motor learning and coordination in the transgenic mice. Prior to synapse disassembly, the overexpression of neuronal Nogo-A led to the downregulation of the synaptic scaffold proteins spectrin, spectrin-E and beta-catenin in the postsynaptic neurons. Our data suggest that neuronal Nogo-A might play a role in the maintenance of inhibitory synapses by modulating the expression of synaptic anchoring molecules.

Neuronal signal integration as well as synaptic transmission and plasticity highly depend on the morphology of dendrites and their spines. Nogo-A is a membrane protein enriched in the adult central nervous system (CNS) myelin, where it restricts the capacity of axons to grow and regenerate after injury. Nogo-A is also expressed by certain neurons, in particular during development, but its physiological function in this cell type is less well understood. We addressed this question in the cerebellum, where Nogo-A is transitorily highly expressed in the Purkinje cells (PCs) during early postnatal development. We used general genetic ablation (KO) as well as selective overexpression of Nogo-A in PCs to analyze its effect on dendritogenesis and on the formation of their main input synapses from parallel (PFs) and climbing fibers (CFs). PC dendritic trees were larger and more complex in Nogo-A KO mice and smaller than in wild-type in Nogo-A overexpressing PCs. Nogo-A KO resulted in premature soma-to-dendrite translocation of CFs and an enlargement of the CF territory in the molecular layer during development. Although spine density was not influenced by Nogo-A, the size of postsynaptic densities of PF-PC synapses was negatively correlated with the Nogo-A expression level. Electrophysiological studies revealed that Nogo-A negatively regulates the strength of synaptic transmission at the PF-PC synapse. Thus, Nogo-A appears as a negative regulator of PC input synapses, which orchestrates cerebellar connectivity through regulation of synapse morphology and the size of the PC dendritic tree.

The membrane protein Nogo‐A inhibits neurite outgrowth and regeneration in the injured central nervous system, primarily because of its expression in oligodendrocytes. Hence, deletion of Nogo‐A enhances regeneration following spinal cord injury. Yet, the effects of Nogo‐A deletion on general behavior and cognition have not been explored. The possibility of potential novel functions of Nogo‐A beyond growth inhibition is strongly suggested by the presence of subpopulations of neurons also expressing Nogo‐A – not only during development but also in adulthood. We evaluated here Nogo‐A−/− mice in a series of general basic behavioral assays as well as functional analyses related to brain regions with notable expression levels of Nogo‐A. The SHIRPA protocol did not show any major basic behavioral changes in Nogo‐A−/− mice. Anxiety‐related behavior, pain sensitivity, startle reactivity, spatial learning, and associative learning also appeared indistinguishable between Nogo‐A−/− and control Nogo‐A+/+ mice. However, motor co‐ordination and balance were enhanced in Nogo‐A−/− mice. Spontaneous locomotor activity was also elevated in Nogo‐A−/− mice, but this was specifically observed in the dark (active) phase of the circadian cycle. Enhanced locomotor reaction to systemic amphetamine in Nogo‐A−/− mice further pointed to an altered dopaminergic tone in these mice. The present study is the first behavioral characterization of mice lacking Nogo‐A and provides significant insights into the potential behavioral relevance of Nogo‐A in the modulation of dopaminergic and motor functions.