Roles of BLOC-1 and Adaptor Protein-3 Complexes in Cargo Sorting to Synaptic Vesicles

Abstract

Neuronal lysosomes and their biogenesis mechanisms are primarily thought to clear metabolites and proteins whose abnormal accumulation leads to neurodegenerative disease pathology. However, it remains unknown whether lysosomal sorting mechanisms regulate the levels of membrane proteins within synaptic vesicles. Using high-resolution deconvolution microscopy, we identified early endosomal compartments where both selected synaptic vesicle and lysosomal membrane proteins coexist with the adaptor protein complex 3 (AP-3) in neuronal cells. From these early endosomes, both synaptic vesicle membrane proteins and characteristic AP-3 lysosomal cargoes can be similarly sorted to brain synaptic vesicles and PC12 synaptic-like microvesicles. Mouse knockouts for two Hermansky-Pudlak complexes involved in lysosomal biogenesis from early endosomes, the ubiquitous isoform of AP-3 (Ap3b1(-/-)) and muted, defective in the biogenesis of lysosome-related organelles complex 1 (BLOC-1), increased the content of characteristic synaptic vesicle proteins and known AP-3 lysosomal proteins in isolated synaptic vesicle fractions. These phenotypes contrast with those of the mouse knockout for the neuronal AP-3 isoform involved in synaptic vesicle biogenesis (Ap3b2(-/-)), in which the content of select proteins was reduced in synaptic vesicles. Our results demonstrate that lysosomal and lysosome-related organelle biogenesis mechanisms regulate steady-state synaptic vesicle protein composition from shared early endosomes.

Phosphoinositides: Tiny Lipids With Giant Impact on Cell Regulation

“…The type II PI4Ks were implicated in transferrin receptor endocytosis and recycling (83), and PI4KII␣ has a major role in EGF receptor degradation in the late endosomal pathway (1060). Recent studies showed that knock-down of type II PI4K␣ causes enlargement of prelysosomal/lysosomal compartments (298, 741,1128), and PI4K␣ is required for lysosomal delivery of the Gaucher disease enzyme ␤-glucocerebrosidase and its receptor, LIMP-2 (741). It will be important to understand where and how the PtdIns4P to PtdIns3P switch happens in the various endocytic compartments.…”

Synaptic Vesicle Endocytosis

“…Thus, if coats are involved, such coats should be different from endocytic clathrin coats. AP-3 and AP-1, heterotetrameric complexes similar to AP-2 that may function independently of clathrin, are potential candidates (Faundez et al 1998;Nakatsu et al 2004;Kim and Ryan 2009;Newell-Litwa et al 2009;Glyvuk et al 2010). However, although both AP-3 and AP-1 were implicated in synaptic vesicle recycling …”

Defective release of α granule and lysosome contents from platelets in mouse Hermansky-Pudlak syndrome models

“…11 Because BLOC-1 and AP-3 regulate SNARE distribution in neurons, [62][63][64] we tested whether these SNARE components were depleted from HPS model platelets. However, levels of each SNARE component were not significantly different in lysates from WT, pearl, pallid, or light ear platelets ( Figure 6E).…”