A Role for Synaptotagmin VII-Regulated Exocytosis of Lysosomes in Neurite Outgrowth from Primary Sympathetic Neurons

Abstract

Neurite outgrowth is mediated by the exocytosis of intracellular vesicles at the tips of elongating neuronal processes. The lysosomal vesicle-associated soluble N-ethylmaleimide-sensitive factor attachment protein receptor tetanus neurotoxin insensitive vesicleassociated membrane protein (TI-VAMP)/VAMP7 was previously implicated in membrane fusion events mediating neurite outgrowth, but the participation of lysosomes in this exocytic process has remained unclear. Here, we show that VAMP7 and the lysosomal glycoprotein Lamp1 extensively colocalize in vesicles present throughout the soma and neurite outgrowths of primary sympathetic neurons. Synaptotagmin VII (Syt VII), a Ca 2ϩ -sensing synaptotagmin isoform previously shown to interact with VAMP7 during lysosomal exocytosis in fibroblasts, was detected on a subset of these lysosomal glycoprotein 1 (Lamp1)/VAMP7-positive neuronal vesicles. Ionophore-stimulated exocytosis triggered exposure of the luminal domains of both Lamp1 and Syt VII at overlapping sites on the neuronal surface, indicating that the Syt VII-containing lysosomal compartments fuse with the plasma membrane in response to [Ca 2ϩ ] i elevation. To determine whether Syt VII was required for the exocytic events mediating neurite extension, we followed the development of superior cervical ganglion neurons explanted from Syt VII-deficient mice. The results revealed a marked defect in neurite outgrowth and arborization, suggesting that Ca 2ϩ -dependent, Syt VII-regulated exocytosis of late endosomes/lysosomes plays a role in the addition of new membrane to developing neurite extensions.

δ-Tocopherol Reduces Lipid Accumulation in Niemann-Pick Type C1 and Wolman Cholesterol Storage Disorders

“…In response to a transient increase of cytosolic Ca 2ϩ , secretory vesicles move toward the plasma membrane, fuse with the membrane, and then expel the luminal contents into the external cellular environment. Exocytosis in nonsecretory cells plays an important role in plasma membrane repair (31), bone resorption (30), cycling/recycling proteins to plasma membrane (32), pathogen invasion (33), neurite outgrowth (34), and cellular clearance (29,35). Lysosomes, which are the most important exocytic organelle in nonsecretory cells, release their contents in response to a transient rise in cytosolic Ca 2ϩ or ionomycin (29).…”

Transport of the Major Myelin Proteolipid Protein Is Directed by VAMP3 and VAMP7

“…We thus propose that VAMP7 mediates regulated exocytosis of LE/Lys in response to neuronal signaling, accounting for recycling of PLP and its inclusion into the myelin domain of the oligodendroglial cell surface. The specific role of VAMP7 in regulated exocytosis of LE/Lys is well established in several cell types (Martinez-Arca et al, 2001;Braun et al, 2004;Rao et al, 2004;Arantes and Andrews, 2006;Logan et al, 2006;ProuxGillardeaux et al, 2007;Marcet-Palacios et al, 2008) and appears primarily associated with cellular processes requiring rapid expansion and remodeling of the plasma membrane (Chaineau et al, 2009;Danglot et al, 2010).…”

Lack of the endosomal SNAREs vti1a and vti1b led to significant impairments in neuronal development

“…In axonal outgrowth the plasma membrane of growth cones is expanded by fusion of membranes derived from different compartments (Fig. S9B): directly from the TGN, by transcytosis (e.g., of L1) via early endosomes, or from lysosomes (31)(32)(33). Endocytosis is required for growth cone dynamics and thus for neurite outgrowth (34,35).…”