Organization of Presynaptic Autophagy-Related Processes

Abstract

Brain synapses pose special challenges on the quality control of their protein machineries as they are far away from the neuronal soma, display a high potential for plastic adaptation and have a high energy demand to fulfill their physiological tasks. This applies in particular to the presynaptic part where neurotransmitter is released from synaptic vesicles, which in turn have to be recycled and refilled in a complex membrane trafficking cycle. Pathways to remove outdated and damaged proteins include the ubiquitin-proteasome system acting in the cytoplasm as well as membrane-associated endolysosomal and the autophagy systems. Here we focus on the latter systems and review what is known about the spatial organization of autophagy and endolysomal processes within the presynapse. We provide an inventory of which components of these degradative systems were found to be present in presynaptic boutons and where they might be anchored to the presynaptic apparatus. We identify three presynaptic structures reported to interact with known constituents of membrane-based protein-degradation pathways and therefore may serve as docking stations. These are (i) scaffolding proteins of the cytomatrix at the active zone, such as Bassoon or Clarinet, (ii) the endocytic machinery localized mainly at the peri-active zone, and (iii) synaptic vesicles. Finally, we sketch scenarios, how presynaptic autophagic cargos are tagged and recruited and which cellular mechanisms may govern membrane-associated protein turnover in the presynapse.

Keywords: Bassoon; active zone (AZ); amphisome; autophagy; endocytic zone; endolysosomal system; presynaptic proteostasis; synaptic vesicle (SV).

FIGURE 1

Major pathways of cellular proteostasis. (A) Major degradative pathways at the presynapse include proteasomal degradation, endo-lysosomal degradation and different types of autophagy. (B) Schematic representation of autophagosome formation and cargo recruitment into autophagosomes via autophagy receptors such as p62/SQMTS1 or ATL3. CMA, chaperone-mediated autophagy; ER, endoplasmic reticulum; PAS, phagophore assembly site (also designated as pre-autophagosomal structure); SVs, synaptic vesicles.

FIGURE 2

Sunburst plots of gene enrichment analyses for autophagy-related genes/proteins included in SynGO (Koopmans et al., 2019). Significantly enriched cellular components (A) and biological processes (B) are indicated by color code. The top-level terms of the Gene Ontology (GO) term tree are represented by the inner circle; the second level of the term tree is denoted by the innermost ring and so on. Presynaptic structures (A) and processes, like synaptic vesicle (SV) cycle and SV endocytosis (B) are significantly over-represented. Note, autophagy-related terms were not annotated in the database. Database entries for cellular components and for biological processes were considered as indicated in Table 1.

FIGURE 3

Autophagy-related proteins detected in the hidden proteome of synaptic vesicles (SV) (Taoufiq et al., 2020). Proteins of the SV-resident repertoire are indicated in different shades of orange; proteins defined by the authors as SV-visitors are gray-shaded. Note, the large active zone scaffolding protein Bassoon may be anchored to SVs via N-terminal myristoylation (Dresbach et al., 2003).

FIGURE 4

Inventory of autophagy-related proteins detected in presynaptic boutons and their relevant sites of action (for details see text and Table 1). Bsn, Bassoon; CAZ, cytomatrix at the active zone; EE, early endosome; ER, endoplasmic reticulum; LE/MVB, late endosome/multi-vesicular body; Lys, lysosome; PAS, phagophore assembly site/pre-autophagosomal structure; Pclo, Piccolo; RE, recycling endosome; SE, sorting endosome; and Bulk indicates bulk endocytosis.

FIGURE 5

Scenario for the regulation of presynaptic autophagy. Autophagy within presynaptic boutons appears to be locally regulated and mediated via the convergence of two major facets of autophagy. (1) Local tagging of aged and/or damaged proteins/organelles by the ubiquitination system. The active zone protein Bassoon is one regulator of this process by scaffolding E3 ligases such as Siah1 and Parkin. Bassoon can also control the induction of phagophore formation by inhibiting the activity of ATG5 and proteasome function via its binding to Psmb4 proteasomal subunit. (2) The formation of phagophore membranes, which requires an interplay between numerous proteins essential for the regulated conjugation of ATG8/LC3 to membranes containing the integral membrane protein ATG9. Many of these proteins are part of the hidden proteome of SVs (see Figure 3) and thus available for the rapid production of these membranes. This aspect of autophagy seems to be coupled to synaptic activity and the sorting and recycling of SV proteins through early endosomes. While not well understood, this compartment is well positioned to not only sort healthy ensembles of proteins regenerating functional SVs, but also damaged ubiquitinated proteins for engulfment into newly forming phagophores. This latter step requires the small GTPase Rab26 and its guanine exchange factor PLEKHG5, as well as autophagy adaptor proteins, like p62/SQSTM1, which binds both poly-ubiquitin chains and ATG8s. The inserted electron micrograph demonstrates the uptake of entire SV-like structures (asterisk) into autophagic vacuoles (arrowhead) within a presynapse (taken from Hoffmann-Conaway et al., 2020; size bar, 50 nm). Finally, the sorting/recycling endosomes also appears to function in the regeneration of SV-like membranes that carry ATG9, providing autophagic support for boutons in subsequent rounds of neurotransmitter release.