AP-1/σ1B-Dependent SV Protein Recycling Is Regulated in Early Endosomes and Is Coupled to AP-2 Endocytosis

Abstract

Adaptor protein (AP)-1/σ1B(-/-) mice have reduced synaptic-vesicle (SV) recycling and increased endosomes. Mutant mice have impaired spatial memory, and σ1B-deficient humans have a severe mental retardation. In order to define these σ1B(-/-) 'bulk' endosomes and to determine their functions in SV recycling, we developed a protocol to separate them from the majority of the neuronal endosomes. The σ1B(-/-) 'bulk' endosomes proved to be classic early endosomes with an increase in the phospholipid phosphatidylinositol 3-phosphate (PI-3-P), which recruits proteins mediating protein sorting out of early endosomes into different routes. σ1B deficiency induced alterations in the endosomal proteome reveals two major functions: SV protein storage and sorting into endolysosomes. Alternative endosomal recycling pathways are not up-regulated, but certain SV proteins are misrouted. Tetraspanins are enriched in σ1B(-/-) synaptosomes, but not in their endosomes or in their clathrin-coated-vesicles (CCVs), indicating AP-1/σ1B-dependent sorting. Synapses contain also more AP-2 CCV, although it is expected that they contain less due to reduced SV recycling. Coat composition of these AP-2 CCVs is altered, and thus, they represent a subpopulation of AP-2 CCVs. Association of calmodulin-dependent protein kinase (CaMK)-IIα, -δ and casein kinase (CK)-IIα with the endosome/SV pool is altered, as well as 14-3-3η, indicating changes in specific signalling pathways regulating synaptic plasticity. The accumulation of early endosomes and endocytotic AP-2 CCV indicates the regulation of SV recycling via early endosomes by the interdependent regulation of AP-2-mediated endocytosis and AP-1/σ1B-mediated SV reformation.

Dual role of the Toxoplasma gondii clathrin adaptor AP1 in the sorting of rhoptry and microneme proteins and in parasite division

“…At the molecular level, AP1 and GGA1 proteins have been demonstrated to regulate Rab5 membrane dynamics by binding directly to the Rab5 effector Rabaptin-5 in mammalian cells [56] [57]. Notably, over-expression of Rabaptin-5 shifts the localization of GGA1- and TGN-associated cargos into enlarged Rab5 endosomes [56] [57] [58]. Therefore, further studies are needed to explore a putative role of Tg AP1 in the regulation of the Rab5 membrane dynamics in T .…”

VGLUT2 Trafficking Is Differentially Regulated by Adaptor Proteins AP-1 and AP-3

“…Indeed, previous studies show that depletion of AP-1 either by genetic ablation or shRNA-mediated silencing of gene expression impairs SV reformation from endosomal compartments ( Glyvuk et al, 2010 ; Cheung and Cousin, 2012 ). Recent biochemical analyses in mice lacking AP-1 further show that trafficking of SV proteins to the RP is impaired, and that AP-1 and AP-2 recycling pathways are interdependent ( Kratzke et al, 2015 ).…”

Synaptic Vesicle Generation from Central Nerve Terminal Endosomes

“…The most likely explanation for this dual requirement is that SV cargo may traffic via a discrete intracellular compartment after leaving the bulk endosome, the most likely candidate being an early/sorting endosome. In support, a recent biochemical study that attempted to purify bulk endosomes from mouse cortex containing a genomic deletion of the AP‐1σB subunit (in which the number of bulk endosomes were increased on stimulation) found an increased prevalence of the early endosome markers Rab5 and early endosomal antigen 1 in endosome fractions . Evidence for the classical endosomal trafficking machinery contributing to SV recycling is still relatively scarce; however, a number of studies have highlighted that SV cargo can traffic via endosomal intermediates that are distinct from bulk endosomes .…”