As the sites of communication between neurons, synapses depend upon precisely regulated protein-protein interactions to support neurotransmitter release and reception. Moreover, neuronal synapses typically exist great distances (i.e. up to meters) away from cell bodies, which are the sources of new proteins and the major sites of protein degradation via lysosomes. Thus, synapses are uniquely sensitive to disruptions in proteostasis, and depend upon carefully orchestrated degradative mechanisms for the clearance of dysfunctional proteins. One of the primary cellular degradative pathways is macroautophagy, hereafter referred to as 'autophagy'. Although it has only recently become a focus of research in synaptic biology, emerging studies indicate that autophagy has essential functions at the synapse throughout an organism's lifetime. This review will discuss recent findings about the roles of synaptic autophagy, as well as some of the questions and issues to be considered in this field moving forward.

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Integration host factor (IHF) is a nucleoid-associated protein involved in DNA packaging, integration of viral DNA and recombination. IHF binds with nanomolar affinity to duplex DNA containing a 13 bp consensus sequence, inducing a bend of ~160° upon binding. We determined that IHF binds to DNA Four-way or Holliday junctions (HJ) with high affinity regardless of the presence of the consensus sequence, signifying a structure-based mechanism of recognition. Junctions, important intermediates in DNA repair and homologous recombination, are dynamic and can adopt either an open or stacked conformation, where the open conformation facilitates branch migration and strand exchange. Using ensemble and single molecule Förster resonance energy transfer (FRET) methods, we investigated IHF-induced changes in the population distribution of junction conformations and determined that IHF binding shifts the population to the open conformation. Further analysis of smFRET dynamics revealed that even in the presence of protein, the junctions remain dynamic as fast transitions are observed for the protein-bound open state. Protein binding alters junction conformational dynamics, as cross correlation analyses reveal the protein slows the transition rate at 1 mM Mg2+ but accelerates the transition rate at 10 mM Mg2+. Stopped flow kinetic experiments provide evidence for two binding steps, a rapid, initial binding step followed by a slower step potentially associated with a conformational change. These measurements also confirm that the protein remains bound to the junction during the conformer transitions and further suggest that the protein forms a partially dissociated state that allows junction arms to be dynamic. These findings, which demonstrate that IHF binds HJs with high affinity and stabilizes junctions in the open conformation, suggest that IHF may play multiple roles in the processes of integration and recombination in addition to stabilizing bacterial biofilms.

Turnover of synaptic vesicle (SV) proteins is vital for the maintenance of healthy, functional synapses in neurons. Our previous work showed that the degradation of SV proteins is mediated by the endosomal sorting complex required for transport (ESCRT) pathway in an activity-dependent manner. Here, we characterize the axonal transport dynamics of ESCRT-0 proteins Hrs and STAM1, the first components of the ESCRT pathway critical for initiating SV protein degradation. Hrs-and STAM1-positive transport vesicles exhibit increased anterograde and bidirectional motility in response to neuronal firing, and frequent colocalization with SV pools. These ESCRT-0 vesicles are a subset of Rab5-positive structures in axons, likely representing pro-degradative early endosomes. Further, we identify kinesin motor protein KIF13A as essential for the activity-dependent transport of ESCRT-0 vesicles as well as the degradation of SV membrane proteins. Together, these data demonstrate a novel KIF13Adependent mechanism for mobilizing axonal transport of ESCRT machinery to facilitate the degradation of SV proteins.Journal 32 (7): 954-69. https://doi.