A novel role for neuritin (cpg15) : the regulation of neuronal migration

Neuritin (Nrn1 or cpg15) is a GPI-anchored protein, originally identified as a candidate plasticity-related gene associated with neurite elongation in several areas of the nervous system. Nrn1 expression is highly up-regulated by neuronal activity, and neurotrophins (BDNF and NT-3). Nrn1 acts in a non-cell autonomous manner to coordinately regulate growth of apposing dendritic and axonal arbors, and to promote synaptic maturation. During PC12 cells differentiation, NGF up-regulates Nrn1 expression, and this enhances the differentiating effects of NGF on PC12 cells favoring the extension of longer neuritis. It has therefore been hypothesized that Nrn1 acts as a downstream effector in promoting neurite outgrowth. Interestingly, Nrn1 expression peaks during neuronal development and particularly in the embryonic proliferative zones. These areas are characterized by an active neuronal migration, a critical event in the development of the nervous system. Neurons and precursor cells migrate using molecular mechanisms which often resembles those involved in neurite outgrowth. Cytoskeletal dynamics, cell adhesion, and chemoattraction/repulsion are all involved both in neuronal migration and neurite extension. A preliminary microarray analysis performed on Gn11 neurons (a model widely utilized to study neuronal migration) showed Nrn1 as one of the more abundant genes expressed in this cell line. Therefore, our goal was to investigate whether Nrn1 regulates neuronal migration. This was performed selecting two similar cell lines, characterized by a different migratory behavior: Gn11 cells (migrating neurons) and GT1-7 cells (non migrating neurons), and analysing their Nrn1 mRNA and protein expression levels. Interestingly, Nrn1 expression is much higher in migrating Gn11 neurons than in non-migrating GT1-7 neurons. In Boyden microchemotaxis and wound-healing assays, the migratory ability of Gn11 neurons is reduced when Nrn1 expression is silenced, while it is increased when Nrn1 is over-expressed. ICC and Western blot analyses of post-translational modifications of tubulin known to be associated with differently dynamic microtubules show an enrichment of stable microtubules in Nrn1 silenced neurons that likely reflects the diminished migratory capability of the cells. These results demonstrate that the migratory properties of Gn11 cells may be altered by modulating Nrn1 levels and strongly suggest, for the first time, that Nrn1 is a regulator of neuronal migration.