Parkinson's disease (PD) and related familial Parkinsonism are defined by motor dysfunction, but the specific upstream molecular causes of these clinical symptoms can vary widely. We hypothesize that these causes converge onto a limited number of core cellular pathways. To investigate this, we created a collection of 24 genetically well-controlled Drosophila models of familial forms of PD and related mono-genic forms of Parkinsonism. Using unbiased behavioral screening and machine learning we identify clusters of mutants that converge on (1) mitochondrial function; (2) retromer/vesicle trafficking and proteostasis/autophagy. Genes within each cluster have a similar genetic interaction profile and compounds that target specific molecular pathways ameliorate dopaminergic neuron dysfunction in a cluster-specific manner. Together, our data indicate that familial PD and related forms of Parkinsonism may fall into two broad functional groups, and may inform further work toward targeted biomarker discovery and therapeutic development.

Abstract Pathogenic variants in ATP13A2 , which encodes an endolysosomal polyamine exporter, cause Kufor-Rakeb syndrome and are associated with early-onset parkinsonism and related neurodegenerative disorders; however, the mechanisms by which ATP13A2 dysfunction drives disease remain incompletely defined. In Atp13a2 knockout mice, we identified an early, transient reduction in brain polyamines that precedes overt gliosis and behavioural abnormalities. Pharmacological polyamine depletion exacerbates phenotypes, whereas oral supplementation of spermidine, but not spermine, rescues parkinsonian symptoms establishing metabolic polyamine deficiency as a pathogenic driver. Mechanistically, spermidine counteracts microglia lysosomal dysfunction in the brain and exerts mitochondrial antioxidant and anti-inflammatory effects in primary mouse microglia, thereby improving neuronal integrity. In the absence of Atp13a2, microglial spermidine import relies on the related polyamine transporter Atp13a3. Importantly, these findings translate to human systems, whereby spermidine attenuates inflammation in ATP13A2-deficient human differentiated microglia, while postmortem ATP13A2-deficient brain analysis confirms increased microglia reactivity. Spermidine also rescues motor deficits and dopaminergic neuron loss in ATP13A2-deficient Drosophila and other fly parkinsonism models. Together, these findings identify early polyamine dysregulation as a mechanistic contributor to ATP13A2-associated parkinsonism and nominate spermidine supplementation as a potential therapeutic strategy for ATP13A2-driven pathology and possibly a broader range of parkinsonian sub-types.

Synaptic neurotransmission is a critical hallmark of brain activity and one of the first processes affected in neural diseases. Monitoring this process, particularly synaptic vesicle recycling, in living cells has been instrumental in revealing the mechanisms responsible for neurotransmitter release. However, currently available reporters suffer from limitations, such as large probe sizes or limited compatibility for human neurons, hampering the quantitative analysis of synaptic pathophysiology. Here, we describe the NbLumSyt1 toolkit, a panel of nanobody-based affinity probes that target the luminal domain of the synaptic vesicle protein Synaptotagmin 1 (Syt1). These new tools enable quantitative, noninvasive imaging and functional interrogation of Syt1 exo-endocytosis and trafficking in human neurons, with unprecedented precision, versatility and cost efficiency, in technologies ranging from fixed- and live-cell super-resolution imaging to electron microscopy and mass spectrometry. Overall, NbLumSyt1 nanobinders provide a valuable platform for studying synaptic physiology and pathophysiology, benefiting fundamental neuroscience and translational efforts to study and develop treatments for brain-related disorders.

Synaptic neurotransmission is a critical hallmark of brain activity and one of the first processes to be affected in neural diseases. Monitoring this process, and in particular synaptic vesicle recycling, in living cells has been instrumental in unraveling mechanisms responsible for neurotransmitter release. However, currently available reporters suffer from major limitations such large probe size or lack of suitability for human neurons, hampering the understanding of human synaptic pathophysiology. Here we describe the NbLumSyt1 toolkit, a panel of nanobody-based affinity probes targeting the luminal domain of the synaptic vesicle protein Synaptotagmin 1 (Syt1). These new tools enable quantitative, non-invasive imaging and functional interrogation of synaptic transmission in human neurons, with unprecedented precision, versatility and cost efficiency, in technologies ranging from fixed-and live-cell super-resolution imaging to electron microscopy and mass spectrometry. Overall, NbLumSyt1 nanobinders provide a valuable platform for human synaptic physiology and pathophysiology, benefiting fundamental neuroscience and translational efforts to study and develop treatments for brain-related disorders. ### Competing Interest Statement Silvio Rizzoli and Felipe Opazo are shareholders of NanoTag Biotechnology GmbH.

This protocol describes how to perform the Startle induced negative geotaxis (SING) assay. SING defects in parkinsonism fly models have previously been shown to be caused by dopaminergic impairments.

This protocol is used to visualize for dopaminergic neurons in Drosophila brains, but can be adapted for other targets as well.

This protocol allows to assess the susceptibility of flies to seizure-like behavior based on mechanicallyhyper-stimulating sensory inputs and assessing seizure-like activity.

The minor phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 ] is crucial for neurotransmission and has been implicated in Parkinson's disease. Here, we present a staining protocol for the analysis of activity -dependent changes of PI(4,5)P 2 at synapses. We describe steps for stimulating and fixing murine hippocampal neurons, staining with probes for PI(4,5)P 2 and a synaptic marker, and analysis by high -resolution microscopy. Our approach gives insights into local PI(4,5)P 2 synthesis and turnover at synapses and can be extended to phosphoinositide lipids other than PI(4,5)P 2 . For complete details on the use and execution of this protocol, please refer to Bolz et al. 1

This protocols describes the recording of electroretinograms (ERGs) and calculation of genetic interactions of fly mutants for a transheterozygous screen.

This protocol describes the preparation of fly food with small molecules to treat flies during their aging. It can be adapted for any treatments. Details for small molecules used can be found in the "Materials" section

This protocol describes the practical steps to perform behavioral monitoring using ethoscopes. The cleaning and preparation of glass tubes as well as loading the flies into ethoscopes is outlined.

Parkinsonism is defined by motor dysfunction, but the specific upstream molecular causes of these clinical symptoms can vary widely. We hypothesize that these causes converge onto a limited number of core cellular pathways. To investigate this, we created a new collection of 24 genetically very well-controlled animal models of familial forms of parkinsonism. Using unbiased behavioral screening and machine learning we identified three clusters of mutants that converge on (1) mitochondrial function; (2) retromer/vesicle trafficking; and (3) proteostasis/autophagy. Genes within each cluster have a similar genetic interaction profile and compounds that target specific molecular pathways ameliorate dopaminergic neuron dysfunction in a cluster-specific manner. This suggests that familial parkinsonism can be stratified into three broad functional groups and our findings pave the way for targeted biomarker discovery and drug development. ### Competing Interest Statement The authors have declared no competing interest.

Synaptic dysfunction is recognized as an early step in the pathophysiology of parkinsonism. Several genetic mutations affecting the integrity of synaptic proteins cause or increase the risk of developing disease. We have identified a candidate causative mutation in synaptic “SH3GL2 Interacting Protein 1” (SGIP1), linked to early-onset parkinsonism in a consanguineous Arab family. Additionally, affected siblings display intellectual, cognitive, and behavioral dysfunction. Metabolic network analysis of [18F]-fluorodeoxyglucose positron emission tomography scans shows patterns very similar to those of idiopathic Parkinson’s disease. We show that the identified SGIP1 mutation causes a loss of protein function, and analyses in newly created Drosophila models reveal movement defects, synaptic transmission dysfunction, and neurodegeneration, including dopaminergic synapse loss. Histology and correlative light and electron microscopy reveal the absence of synaptic multivesicular bodies and the accumulation of degradative organelles. This research delineates a putative form of recessive parkinsonism, converging on defective synaptic proteostasis and opening avenues for diagnosis, genetic counseling, and treatment.

Pathogenic α-synuclein and tau are critical drivers of neurodegeneration and their mutations cause neuronal loss in patients. Whether the underlying preferential neuronal vulnerability is a cell-type intrinsic property or a consequence of increased expression levels is an open question. Here, we explore cell-type specific α-synuclein and tau expression in human brain datasets and use deep phenotyping as well as brain-wide single-cell RNA sequencing of >200 live neuron types in fruit flies to ask which cellular environments react most to α-synuclein or tau toxicity. We detect phenotypic and transcriptomic evidence of differential neuronal vulnerability independent of α-synuclein or tau expression levels. Comparing vulnerable with resilient neurons enabled us to identify molecular signatures associated with these differential responses. We used these to verify, and then predict resilient and vulnerable neuron subtypes in human brains. This confirms substantia nigra dopaminergic neurons to be sensitive to α-synuclein, and we predict pathogenic tau vulnerable and protected cortical neuron subtypes. Our work indicates that cellular determinants confer selective vulnerability to specific types of amyloid toxicity, thus paving the way to leverage neuronal identity to uncover modifiers of neurodegeneration-associated toxic proteins.

Exocytosis and endocytosis are essential physiological processes and are of prime importance for brain function. Neurotransmission depends on the Ca2+-triggered exocytosis of synaptic vesicles (SVs). In neurons, exocytosis is spatiotemporally coupled to the retrieval of an equal amount of membrane and SV proteins by compensatory endocytosis. How exocytosis and endocytosis are balanced to maintain presynaptic membrane homeostasis and, thereby, sustain brain function is essentially unknown. We combine mouse genetics with optical imaging to show that the SV calcium sensor Synaptotagmin 1 couples exocytic SV fusion to the endocytic retrieval of SV membranes by promoting the local activity-dependent formation of the signaling lipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at presynaptic sites. Interference with these mechanisms impairs PI(4,5)P2-triggered SV membrane retrieval but not exocytic SV fusion. Our findings demonstrate that the coupling of SV exocytosis and endocytosis involves local Synaptotagmin 1-induced lipid signaling to maintain presynaptic membrane homeostasis in central nervous system neurons.

Neuronal networks maintain stable activity around a given set point, an enigmatic variable in homeostatic systems. In this issue of Neuron, Styr et al. (2019) now show that set points are regulated by mitochondria and propose a potential strategy to treat refractory forms of epilepsy.

Neurotransmission is based on the exocytic fusion of synaptic vesicles (SVs) followed by endocytic membrane retrieval and the reformation of SVs. Recent data suggest that at physiological temperature SVs are internalized via clathrin-independent ultrafast endocytosis (UFE) within hundreds of milliseconds, while other studies have postulated a key role for clathrin-mediated endocytosis (CME) of SV proteins on a timescale of seconds to tens of seconds. Here we demonstrate using cultured hippocampal neurons as a model that at physiological temperature SV endocytosis occurs on several timescales from less than a second to several seconds, yet, is largely independent of clathrin. Clathrin-independent endocytosis (CIE) of SV membranes is mediated by actin-nucleating formins such as mDia1, which are required for the formation of presynaptic endosome-like vacuoles from which SVs reform. Our results resolve previous discrepancies in the field and suggest that SV membranes are predominantly retrieved via CIE mediated by formin-dependent actin assembly.

Communication between neurons relies on neurotransmitters which are released from synaptic vesicles (SVs) upon Ca2+ stimuli. To efficiently load neurotransmitters, sense the rise in intracellular Ca2+ and fuse with the presynaptic membrane, SVs need to be equipped with a stringently controlled set of transmembrane proteins. In fact, changes in SV protein composition quickly compromise neurotransmission and most prominently give rise to epileptic seizures. During exocytosis SVs fully collapse into the presynaptic membrane and consequently have to be replenished to sustain neurotransmission. Therefore, surface-stranded SV proteins have to be efficiently retrieved post-fusion to be used for the generation of a new set of fully functional SVs, a process in which dedicated endocytic sorting adaptors play a crucial role. The question of how the precise reformation of SVs is achieved is intimately linked to how SV membranes are retrieved. For a long time both processes were believed to be two sides of the same coin since Clathrin-mediated endocytosis (CME), the proposed predominant SV recycling mode, will jointly retrieve SV membranes and proteins. However, with the recent proposal of Clathrin-independent SV recycling pathways SV membrane retrieval and SV reformation turn into separable events. This review highlights the progress made in unraveling the molecular mechanisms mediating the high-fidelity retrieval of SV proteins and discusses how the gathered knowledge about SV protein recycling fits in with the new notions of SV membrane endocytosis.

Neurotransmission involves the calcium-regulated exocytic fusion of synaptic vesicles (SVs) and the subsequent retrieval of SV membranes followed by reformation of properly sized and shaped SVs. An unresolved question is whether each SV protein is sorted by its own dedicated adaptor or whether sorting is facilitated by association between different SV proteins. We demonstrate that endocytic sorting of the calcium sensor synaptotagmin 1 (Syt1) is mediated by the overlapping activities of the Syt1-associated SV glycoprotein SV2A/B and the endocytic Syt1-adaptor stonin 2 (Stn2). Deletion or knockdown of either SV2A/B or Stn2 results in partial Syt1 loss and missorting of Syt1 to the neuronal surface, whereas deletion of both SV2A/B and Stn2 dramatically exacerbates this phenotype. Selective missorting and degradation of Syt1 in the absence of SV2A/B and Stn2 impairs the efficacy of neurotransmission at hippocampal synapses. These results indicate that endocytic sorting of Syt1 to SVs is mediated by the overlapping activities of SV2A/B and Stn2 and favor a model according to which SV protein sorting is guarded by both cargo-specific mechanisms as well as association between SV proteins.