A New Form of Neurite Outgrowth Sustained by the Exocytosis of Enlargeosomes Expressed under the Control of REST

In neurons and neurosecretory (nerve) cells, neurite outgrowth requires surface enlargement sustained by exocytosis of specific but poorly characterized vesicles. A canonical, relatively slow form of outgrowth is known to require the v‐SNARE Ti‐VAMP. Recently, we have identified a new, rapid form, triggered by activation of Rac1 and sustained by the exocytosis of enlargeosomes (v‐SNARE: VAMP4). By parallel study of various pheochromocytoma PC12 cell clones exhibiting either a single or both forms of outgrowth, we show that expression of enlargeosomes, their exocytosis at growth cones and their form of neurite outgrowth are positively governed by the RE‐1 silencing transcription factor (REST), a repressor of many nerve cell‐specific genes. Using a high REST/enlargeosome‐rich PC12 clone transfected with TrkA, we found (i) that nerve growth factor (NGF) can increase the expression of both REST and the enlargeosome maker, Ahnak; and (ii) that outgrowth triggered by NGF, independent from the form triggered by Rac1 and supported mostly by exocytic, Ti‐VAMP‐positive organelles distinct from enlargeosomes, occurs at slow or fast rates depending on the strength of the TrkA signaling. These results confirm the duality of the outgrowth forms sustained by the two types of exocytic vesicles, reveal their distinct properties and identify new aspects of the REST impact in nerve cell specificity/function.