No abstract

PINK1 phosphorylates ubiquitin, which then binds to Parkin and activates its E3 ligase activity, leading to induction of selective autophagy of damaged mitochondria.

Synapses are asymmetric cell junctions with precisely juxtaposed presynaptic and postsynaptic sides. Transsynaptic adhesion complexes are thought to organize developing synapses. The molecular composition of these complexes, however, remains incompletely understood, precluding us from understanding how adhesion across the synaptic cleft guides synapse development. Here, we define two immunoglobulin superfamily members, SynCAM 1 and 2, that are expressed in neurons in the developing brain and localize to excitatory and inhibitory synapses. They function as cell adhesion molecules and assemble with each other across the synaptic cleft into a specific, transsynaptic SynCAM 1/2 complex. Additionally, SynCAM 1 and 2 promote functional synapses as they increase the number of active presynaptic terminals and enhance excitatory neurotransmission. The interaction of SynCAM 1 and 2 is affected by glycosylation, indicating regulation of this adhesion complex by posttranslational modification. The SynCAM 1/2 complex is representative for the highly defined adhesive patterns of this protein family, the four members of which are expressed in neurons in divergent expression profiles. SynCAMs 1, 2, and 3 each can bind themselves, yet preferentially assemble into specific, heterophilic complexes as shown for the synaptic SynCAM 1/2 interaction and a second complex comprising SynCAM 3 and 4. Our results define SynCAM proteins as components of novel heterophilic transsynaptic adhesion complexes that set up asymmetric interactions, with SynCAM proteins contributing to synapse organization and function.

Damaged mitochondria can be selectively eliminated by mitophagy. Although two gene products mutated in Parkinson’s disease, PINK1, and Parkin have been found to play a central role in triggering mitophagy in mammals, how the pre-autophagosomal isolation membrane selectively and accurately engulfs damaged mitochondria remains unclear. In this study, we demonstrate that TBC1D15, a mitochondrial Rab GTPase-activating protein (Rab-GAP), governs autophagosome biogenesis and morphology downstream of Parkin activation. To constrain autophagosome morphogenesis to that of the cargo, TBC1D15 inhibits Rab7 activity and associates with both the mitochondria through binding Fis1 and the isolation membrane through the interactions with LC3/GABARAP family members. Another TBC family member TBC1D17, also participates in mitophagy and forms homodimers and heterodimers with TBC1D15. These results demonstrate that TBC1D15 and TBC1D17 mediate proper autophagic encapsulation of mitochondria by regulating Rab7 activity at the interface between mitochondria and isolation membranes.DOI: http://dx.doi.org/10.7554/eLife.01612.001

The transcription factor TFEB is activated in a Parkin- and Atg5-dependent manner during mitophagy, and MiT/TFE transcription factor family members are required for the efficient clearance of damaged mitochondria.

Summary Synaptogenesis is required for wiring neuronal circuits in the developing brain and continues to remodel adult networks. However, the molecules organizing synapse development and maintenance in vivo remain incompletely understood. We now demonstrate that the immunoglobulin adhesion molecule SynCAM 1 dynamically alters synapse number and plasticity. Overexpression of SynCAM 1 in transgenic mice promotes excitatory synapse number, while loss of SynCAM 1 results in fewer excitatory synapses. By turning off SynCAM 1 overexpression in transgenic brains, we show that it maintains the newly induced synapses. SynCAM 1 also functions at mature synapses to alter their plasticity by regulating long-term depression. Consistent with these effects on neuronal connectivity, SynCAM 1 expression affects spatial learning, with knock-out mice learning better. The reciprocal effects of increased SynCAM 1 expression and loss reveal that this adhesion molecule contributes to the regulation of synapse number and plasticity, and impacts how neuronal networks undergo activity-dependent changes.

Synapses are specialized adhesion sites between neurons that are connected by protein complexes spanning the synaptic cleft. These trans-synaptic interactions can organize synapse formation, but their macromolecular properties and effects on synaptic morphology remain incompletely understood. Here, we demonstrate that the synaptic cell adhesion molecule SynCAM 1 self-assembles laterally via its extracellular, membrane-proximal immunoglobulin (Ig) domains 2 and 3. This cis oligomerization generates SynCAM oligomers with increased adhesive capacity and instructs the interactions of this molecule across the nascent and mature synaptic cleft. In immature neurons, cis assembly promotes the adhesive clustering of SynCAM 1 at new axo-dendritic contacts. Interfering with the lateral self-assembly of SynCAM 1 in differentiating neurons strongly impairs its synaptogenic activity. At later stages, the lateral oligomerization of SynCAM 1 restricts synaptic size, indicating that this adhesion molecule contributes to the structural organization of synapses. These results support that lateral interactions assemble SynCAM complexes within the synaptic cleft to promote synapse induction and modulate their structure. These findings provide novel insights into synapse development and the adhesive mechanisms of Ig superfamily members.

During autophagy, a double membrane envelops cellular material for trafficking to the lysosome. Human beclin-1 and its yeast homologue, Atg6/Vps30, are scaffold proteins bound in a lipid kinase complex with multiple cellular functions, including autophagy. Several different Atg6 complexes exist, with an autophagy-specific form containing Atg14. However, the roles of Atg14 and beclin-1 in the activation of this complex remain unclear. We here addressed the mechanism of beclin-1 complex activation and reveal two critical steps in this pathway. First, we identified a unique domain in beclin-1, conserved in the yeast homologue Atg6, which is involved in membrane association and, unexpectedly, controls autophagosome size and number in yeast. Second, we demonstrated that human Atg14 is critical in controlling an autophagy-dependent phosphorylation of beclin-1. We map these novel phosphorylation sites to serines 90 and 93 and demonstrate that phosphorylation at these sites is necessary for maximal autophagy. These results help clarify the mechanism of beclin-1 and Atg14 during autophagy.A utophagy is a catabolic membrane trafficking process that turns over cytosolic material following encapsulation by autophagosomes and subsequent degradation in the lysosome (in higher eukaryotes) or the vacuole (in plants and unicellular eukaryotes). Genetic studies in yeast have identified a battery of conserved proteins that are required for starvation-induced autophagy (1). In both higher eukaryotes and yeast, autophagy also occurs constitutively as a cargo-selective quality control process (2). Consistent with its dual role in metabolism and cellular quality control, autophagy has been shown to play a role in a variety of human pathologies, including cancer and neurodegeneration (3).Many of the proteins identified as essential for autophagy in yeast have homologues in higher eukaryotes that carry out conserved functions. Among these is the tumor suppressor beclin-1, discovered in a two-hybrid screen as a protein that interacts with the antiapoptotic protein Bcl-2 (4, 5). Beclin-1 is a core component of the phosphatidylinositol 3-kinase complex, along with the catalytic subunit Vps34 and the putative protein kinase Vps15 (6). Atg6/Vps30, the yeast homologue of beclin-1, shares 24.4% amino acid homology and functions in selective and nonselective autophagy (7), as well as endosomal trafficking. Similar to beclin-1, additional components bind the core Atg6-lipid kinase complex to direct functions in these different membrane trafficking pathways (8).Localization of the yeast Atg6/Vps15/Vps34 complex to the preautophagosomal structure is largely dictated by the binding of Atg6 to Atg14 (7). Recently, the human homologue of Atg14, hAtg14/Barkor/Atg14L (here referred to as hAtg14), has been identified by several groups. hAtg14 has been shown to be a member of a beclin-1 complex analogous to that in yeast, although the mechanism of action in autophagy may be distinct (6,9,10,11). hAtg14 interacts with beclin-1 through its coiled-coil doma...