Introducing secretory reticulophagy/ER-phagy (SERP), a VAMP7-dependent pathway involved in neurite growth

Together with the proteasome, macroautophagy is a main pathway for the degradation of intracellular elements. Endoplasmic reticulum (ER)-autophagy i.e. reticulophagy/ER-phagy leads to the encapsulation of pieces of the ER in forming autophagosomes. This is generally followed by fusion with lysosomes and degradation of these ER components by lysosomal hydrolases. Recent work by our group shows that ER elements could also be incorporated into late endosomes and later be released by a secretory mechanism which we will herein refer to as secretory reticulophagy/ER-phagy (SERP). In the absence of macroautophagy, such as by knocking out Atg5, SERP is more efficient, leading to an increased secretion of MAP1LC3B-II and LC3-interacting region (LIR)-containing proteins of the ER, reticulons and atlastins. In this scenario, neurites grow longer and neuronal polarity is altered. In the absence of SERP, such as by knocking out Vamp7, secretion of MAP1LC3B-II, ER-LIR containing proteins and neurite growth are severely inhibited. We argue that SERP might be a main secretory mechanism bypassing the Golgi apparatus, and that it is particularly active and important in neurite growth.