The filamentous structures that tether exocytic vesicles to the plasma membrane in the active zone are rearranged in response to synaptic stimulation.

Summary Protein aggregation and dysfunction of the ubiquitin-proteasome system are hallmarks of many neurodegenerative diseases. Here we address the elusive link between these phenomena employing cryo-electron tomography to dissect the molecular architecture of protein aggregates within intact neurons at high resolution. We focus on the poly-Gly-Ala (GA) aggregates resulting from aberrant translation of an expanded GGGGCC repeat in C9orf72, the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia. We find that poly-GA aggregates consist of densely packed twisted ribbons that recruit numerous 26S proteasome complexes, while other macromolecules are largely excluded. Proximity to poly-GA ribbons stabilizes a transient substrate-processing conformation of the 26S proteasome, suggesting stalled degradation. Thus, poly-GA aggregates may compromise neuronal proteostasis by driving the accumulation and functional impairment of a large fraction of cellular proteasomes.

Expression of many disease-related aggregation-prone proteins results in cytotoxicity and the formation of large intracellular inclusion bodies. To gain insight into the role of inclusions in pathology and the in situ structure of protein aggregates inside cells, we employ advanced cryo-electron tomography methods to analyze the structure of inclusions formed by polyglutamine (polyQ)-expanded huntingtin exon 1 within their intact cellular context. In primary mouse neurons and immortalized human cells, polyQ inclusions consist of amyloid-like fibrils that interact with cellular endomembranes, particularly of the endoplasmic reticulum (ER). Interactions with these fibrils lead to membrane deformation, the local impairment of ER organization, and profound alterations in ER membrane dynamics at the inclusion periphery. These results suggest that aberrant interactions between fibrils and endomembranes contribute to the deleterious cellular effects of protein aggregation. VIDEO ABSTRACT.

The close apposition between the endoplasmic reticulum (ER) and the plasma membrane (PM) plays important roles in Ca 2+ homeostasis, signaling, and lipid metabolism. The extended synaptotagmins (E-Syts; tricalbins in yeast) are ER-anchored proteins that mediate the tethering of the ER to the PM and are thought to mediate lipid transfer between the two membranes. E-Syt cytoplasmic domains comprise a synaptotagmin-like mitochondrial-lipidbinding protein (SMP) domain followed by five C2 domains in E-Syt1 and three C2 domains in E-Syt2/3. Here, we used cryo-electron tomography to study the 3D architecture of E-Syt-mediated ER-PM contacts at molecular resolution. In vitrified frozen-hydrated mammalian cells overexpressing individual E-Syts, in which E-Sytdependent contacts were by far the predominant contacts, ER-PM distance (19-22 nm) correlated with the amino acid length of the cytosolic region of E-Syts (i.e., the number of C2 domains). Elevation of cytosolic Ca 2+ shortened the ER-PM distance at E-Syt1-dependent contacts sites. E-Syt-mediated contacts displayed a characteristic electron-dense layer between the ER and the PM. These features were strikingly different from those observed in cells exposed to conditions that induce contacts mediated by the stromal interaction molecule 1 (STIM1) and the Ca 2+ channel Orai1 as well as store operated Ca 2+ entry. In these cells the gap between the ER and the PM was spanned by filamentous structures perpendicular to the membranes. Our results define specific ultrastructural features of E-Syt-dependent ER-PM contacts and reveal their structural plasticity, which may impact on the cross-talk between the ER and the PM and the functions of E-Syts in lipid transport between the two bilayers.T he endoplasmic reticulum (ER) consists of a complex network of tubules and cisternae that extends throughout the cell and forms close appositions ("contact sites") with other membranous organelles and with the plasma membrane (PM) (1, 2). The best characterized function of ER-PM contacts is in Ca 2+ homeostasis, as ER-PM contacts mediate the excitationcontraction coupling in muscle and store-operated Ca 2+ entry (SOCE) in all metazoan cells. In SOCE, upon depletion of Ca 2+ in the ER, the ER protein stromal interaction molecule 1 (STIM1) oligomerizes, binds, and activates Orai1 Ca 2+ channels at the PM to drive influx of extracellular Ca 2+ , thereby allowing homeostatic regulation of ER Ca 2+ levels (3, 4). However, growing evidence suggests that ER-PM contacts also play more general roles, including signaling (5, 6) and the regulation of both lipid metabolism and transport between bilayers (1, 7-17).We recently have shown that the three mammalian extended synaptotagmins (E-Syts), homologs of the yeast tricalbins (18,19), act as ER-PM tethers (20). E-Syts are ER-anchored proteins (via an N-terminal hairpin) containing a synaptotagmin-like mitochondrial-lipid-binding protein (SMP) domain followed by five (E-Syt1) or three (E-Syt2/3) C2 domains. SMP domains are present in proteins that loc...

SummaryHuntingtin (Htt) is a large (348 kDa) protein, essential for embryonic development and involved in diverse cellular activities such as vesicular transport, endocytosis, autophagy and transcription regulation1,2. While an integrative understanding of Htt's biological functions is lacking, the large number of identified interactors suggests that Htt serves as a protein-protein interaction hub1,3,4. Furthermore, Huntington’s disease is caused by a mutation in the Htt gene, resulting in a pathogenic expansion of a polyglutamine (polyQ) repeat at the N-terminus of Htt5,6. However, only limited structural information on Htt is currently available. Here we employed cryo-electron microscopy (cryo-EM) to determine the structure of full-length human Htt in a complex with HAP40/F8A7 to 4 Å resolution. Htt is largely α-helical and consists of three major domains. The N- and C-terminal domains contain multiple HEAT repeats arranged in a solenoid fashion. These domains are connected by a smaller bridge domain containing different types of tandem repeats. HAP40 is also largely α-helical and has a tetratricopeptide repeat (TPR)-like organization. HAP40 binds in a cleft contacting the three Htt domains by hydrophobic and electrostatic interactions, thereby stabilizing Htt conformation. These data rationalize previous biochemical results and pave the way for an improved understanding of Htt’s diverse cellular functions.

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RIM1α-deficient synapses show structural defects in presynaptic vesicle distribution and tethering to the active zone that can be reversed by proteasome inhibition.

Synucleins (α, β, γ-synuclein) are a family of abundant presynaptic proteins. α-Synuclein is causally linked to the pathogenesis of Parkinson disease (PD). In an effort to define their physiological and pathological function(s), we investigated the effects of deleting synucleins and overexpressing α-synuclein PD mutations, in mice, on synapse architecture using electron microscopy (EM) and Cryo-Electron Tomography (Cryo-ET). We show that synucleins are regulators of presynapse size and synaptic vesicle (SV) pool organization. Using Cryo-ET, we observed that deletion of synucleins increases SV tethering to the active zone but decreases the inter-linking of SVs by short connectors. These ultrastructural changes were correlated with discrete protein phosphorylation changes in αβγ-synuclein−/− neurons. We also determined that α-synuclein PD mutants (PARK1/hA30P, PARK4/hα-syn) primarily affected presynaptic cytomatrix proximal to the active zone, congruent with previous findings that these PD mutations decrease neurotransmission. Collectively, our results suggest that synucleins are important orchestrators of presynaptic terminal topography.