Sabine Kuenen

Loss of Skywalker Reveals Synaptic Endosomes as Sorting Stations for Synaptic Vesicle Proteins

Exchange of proteins at sorting endosomes is not only critical to numerous signaling pathways but also to receptor-mediated signaling and to pathogen entry into cells; however, how this process is regulated in synaptic vesicle cycling remains unexplored. In this work, we present evidence that loss of function of a single neuronally expressed GTPase activating protein (GAP), Skywalker (Sky) facilitates endosomal trafficking of synaptic vesicles at Drosophila neuromuscular junction boutons, chiefly by controlling Rab35 GTPase activity. Analyses of genetic interactions with the ESCRT machinery as well as chimeric ubiquitinated synaptic vesicle proteins indicate that endosomal trafficking facilitates the replacement of dysfunctional synaptic vesicle components. Consequently, sky mutants harbor a larger readily releasable pool of synaptic vesicles and show a dramatic increase in basal neurotransmitter release. Thus, the trafficking of vesicles via endosomes uncovered using sky mutants provides an elegant mechanism by which neurons may regulate synaptic vesicle rejuvenation and neurotransmitter release.

Inactivation of clathrin heavy chain inhibits synaptic recycling but allows bulk membrane uptake

Synaptic vesicle reformation depends on clathrin, an abundant protein that polymerizes around newly forming vesicles. However, how clathrin is involved in synaptic recycling in vivo remains unresolved. We test clathrin function during synaptic endocytosis using clathrin heavy chain (chc) mutants combined with chc photoinactivation to circumvent early embryonic lethality associated with chc mutations in multicellular organisms. Acute inactivation of chc at stimulated synapses leads to substantial membrane internalization visualized by live dye uptake and electron microscopy. However, chc-inactivated membrane cannot recycle and participate in vesicle release, resulting in a dramatic defect in neurotransmission maintenance during intense synaptic activity. Furthermore, inactivation of chc in the context of other endocytic mutations results in membrane uptake. Our data not only indicate that chc is critical for synaptic vesicle recycling but they also show that in the absence of the protein, bulk retrieval mediates massive synaptic membrane internalization.

Recombineering-mediated tagging of Drosophila genomic constructs for in vivo localization and acute protein inactivation

Studying gene function in the post-genome era requires methods to localize and inactivate proteins in a standardized fashion in model organisms. While genome-wide gene disruption and over-expression efforts are well on their way to vastly expand the repertoire of Drosophila tools, a complementary method to efficiently and quickly tag proteins expressed under endogenous control does not exist for fruit flies. Here, we describe the development of an efficient procedure to generate protein fusions at either terminus in an endogenous genomic context using recombineering. We demonstrate that the fluorescent protein tagged constructs, expressed under the proper control of regulatory elements, can rescue the respective mutations and enable the detection of proteins in vivo. Furthermore, we also adapted our method for use of the tetracysteine tag that tightly binds the fluorescent membrane-permeable FlAsH ligand. This technology allows us to acutely inactivate any tagged protein expressed under native control using fluorescein-assisted light inactivation and we provide proof of concept by demonstrating that acute loss of clathrin heavy chain function in the fly eye leads to synaptic transmission defects in photoreceptors. Our tagging technology is efficient and versatile, adaptable to any tag desired and paves the way to genome-wide gene tagging in Drosophila.

Hsc70-4 Deforms Membranes to Promote Synaptic Protein Turnover by Endosomal Microautophagy

Synapses are often far from their cell bodies and must largely independently cope with dysfunctional proteins resulting from synaptic activity and stress. To identify membrane-associated machines that can engulf synaptic targets destined for degradation, we performed a large-scale in vitro liposome-based screen followed by functional studies. We identified a presynaptically enriched chaperone Hsc70-4 that bends membranes based on its ability to oligomerize. This activity promotes endosomal microautophagy and the turnover of specific synaptic proteins. Loss of microautophagy slows down neurotransmission while gain of microautophagy increases neurotransmission. Interestingly, Sgt, a cochaperone of Hsc70-4, is able to switch the activity of Hsc70-4 from synaptic endosomal microautophagy toward chaperone activity. Hence, Hsc70-4 controls rejuvenation of the synaptic protein pool in a dual way: either by refolding proteins together with Sgt, or by targeting them for degradation by facilitating endosomal microautophagy based on its membrane deforming activity.

Reduced synaptic vesicle protein degradation at lysosomes curbs TBC1D24/sky-induced neurodegeneration

Efficient lysosome-mediated turnover of synaptic vesicle-associated proteins is necessary for synaptic transmission and protection from neurodegeneration in Drosophila.

The SAC1 domain in synaptojanin is required for autophagosome maturation at presynaptic terminals

Presynaptic terminals are metabolically active and accrue damage through continuous vesicle cycling. How synapses locally regulate protein homeostasis is poorly understood. We show that the presynaptic lipid phosphatase synaptojanin is required for macroautophagy, and this role is inhibited by the Parkinsons disease mutation R258Q. Synaptojanin drives synaptic endocytosis by dephosphorylating PI(4,5)P2, but this function appears normal in SynaptojaninRQ knock‐in flies. Instead, R258Q affects the synaptojanin SAC1 domain that dephosphorylates PI(3)P and PI(3,5)P2, two lipids found in autophagosomal membranes. Using advanced imaging, we show that SynaptojaninRQ mutants accumulate the PI(3)P/PI(3,5)P2‐binding protein Atg18a on nascent synaptic autophagosomes, blocking autophagosome maturation at fly synapses and in neurites of human patient induced pluripotent stem cell‐derived neurons. Additionally, we observe neurodegeneration, including dopaminergic neuron loss, in SynaptojaninRQ flies. Thus, synaptojanin is essential for macroautophagy within presynaptic terminals, coupling protein turnover with synaptic vesicle cycling and linking presynaptic‐specific autophagy defects to Parkinsons disease.

Dynamin photoinactivation blocks Clathrin and α-adaptin recruitment and induces bulk membrane retrieval

Drosophila Dynamin prevents bulk membrane endocytosis through effects on AP2- and Clathrin-mediated stabilization of endocytic pits.

Mitochondrial uncouplers inhibit clathrin-mediated endocytosis largely through cytoplasmic acidification

ATP production requires the establishment of an electrochemical proton gradient across the inner mitochondrial membrane. Mitochondrial uncouplers dissipate this proton gradient and disrupt numerous cellular processes, including vesicular trafficking, mainly through energy depletion. Here we show that Endosidin9 (ES9), a novel mitochondrial uncoupler, is a potent inhibitor of clathrin-mediated endocytosis (CME) in different systems and that ES9 induces inhibition of CME not because of its effect on cellular ATP, but rather due to its protonophore activity that leads to cytoplasm acidification. We show that the known tyrosine kinase inhibitor tyrphostinA23, which is routinely used to block CME, displays similar properties, thus questioning its use as a specific inhibitor of cargo recognition by the AP-2 adaptor complex via tyrosine motif-based endocytosis signals. Furthermore, we show that cytoplasm acidification dramatically affects the dynamics and recruitment of clathrin and associated adaptors, and leads to reduction of phosphatidylinositol 4,5-biphosphate from the plasma membrane.

Skywalker-TBC1D24 has a lipid-binding pocket mutated in epilepsy and required for synaptic function.

Mutations in TBC1D24 cause severe epilepsy and DOORS syndrome, but the molecular mechanisms underlying these pathologies are unresolved. We solved the crystal structure of the TBC domain of the Drosophila ortholog Skywalker, revealing an unanticipated cationic pocket conserved among TBC1D24 homologs. Cocrystallization and biochemistry showed that this pocket binds phosphoinositides phosphorylated at the 4 and 5 positions. The most prevalent patient mutations affect the phosphoinositide-binding pocket and inhibit lipid binding. Using in vivo photobleaching of Skywalker-GFP mutants, including pathogenic mutants, we showed that membrane binding via this pocket restricts Skywalker diffusion in presynaptic terminals. Additionally, the pathogenic mutations cause severe neurological defects in flies, including impaired synaptic-vesicle trafficking and seizures, and these defects are reversed by genetically increasing synaptic PI(4,5)P2 concentrations through synaptojanin mutations. Hence, we discovered that a TBC domain affected by clinical mutations directly binds phosphoinositides through a cationic pocket and that phosphoinositide binding is critical for presynaptic function.

Dual loss of succinate dehydrogenase (SDH) and complex I activity is necessary to recapitulate the metabolic phenotype of SDH mutant tumors

Mutations in succinate dehydrogenase (SDH) are associated with tumor development and neurodegenerative diseases. Only in tumors, loss of SDH activity is accompanied with the loss of complex I activity. Yet, it remains unknown whether the metabolic phenotype of SDH mutant tumors is driven by loss of complex I function, and whether this contributes to the peculiarity of tumor development versus neurodegeneration. We addressed this question by decoupling loss of SDH and complex I activity in cancer cells and neurons. We found that sole loss of SDH activity was not sufficient to recapitulate the metabolic phenotype of SDH mutant tumors, because it failed to decrease mitochondrial respiration and to activate reductive glutamine metabolism. These metabolic phenotypes were only induced upon the additional loss of complex I activity. Thus, we show that complex I function defines the metabolic differences between SDH mutation associated tumors and neurodegenerative diseases, which could open novel therapeutic options against both diseases.