Benjamin R. Rost

Ultrafast endocytosis at mouse hippocampal synapses

To sustain neurotransmission, synaptic vesicles and their associated proteins must be recycled locally at synapses. Synaptic vesicles are thought to be regenerated approximately 20 s after fusion by the assembly of clathrin scaffolds or in approximately 1 s by the reversal of fusion pores via ‘kiss-and-run’ endocytosis. Here we use optogenetics to stimulate cultured hippocampal neurons with a single stimulus, rapidly freeze them after fixed intervals and examine the ultrastructure using electron microscopy—‘flash-and-freeze’ electron microscopy. Docked vesicles fuse and collapse into the membrane within 30 ms of the stimulus. Compensatory endocytosis occurs within 50 to 100 ms at sites flanking the active zone. Invagination is blocked by inhibition of actin polymerization, and scission is blocked by inhibiting dynamin. Because intact synaptic vesicles are not recovered, this form of recycling is not compatible with kiss-and-run endocytosis; moreover, it is 200-fold faster than clathrin-mediated endocytosis. It is likely that ‘ultrafast endocytosis’ is specialized to restore the surface area of the membrane rapidly.

Role of Amyloid-β Glycine 33 in Oligomerization, Toxicity, and Neuronal Plasticity

The aggregation of the amyloid-β (Aβ) peptide plays a pivotal role in the pathogenesis of Alzheimers disease, as soluble oligomers are intimately linked to neuronal toxicity and inhibition of hippocampal long-term potentiation (LTP). In the C-terminal region of Aβ there are three consecutive GxxxG dimerization motifs, which we could previously demonstrate to play a critical role in the generation of Aβ. Here, we show that glycine 33 (G33) of the central GxxxG interaction motif within the hydrophobic Aβ sequence is important for the aggregation dynamics of the peptide. Aβ peptides with alanine or isoleucine substitutions of G33 displayed an increased propensity to form higher oligomers, which we could attribute to conformational changes. Importantly, the oligomers of G33 variants were much less toxic than Aβ42 wild type (WT), in vitro and in vivo. Also, whereas Aβ42 WT is known to inhibit LTP, Aβ42 G33 variants had lost the potential to inhibit LTP. Our findings reveal that conformational changes induced by G33 substitutions unlink toxicity and oligomerization of Aβ on the molecular level and suggest that G33 is the key amino acid in the toxic activity of Aβ. Thus, a specific toxic conformation of Aβ exists, which represents a promising target for therapeutic interventions.

Loss of classical transient receptor potential 6 channel reduces allergic airway response

Background Non‐selective cation influx through canonical transient receptor potential channels (TRPCs) is thought to be an important event leading to airway inflammation. TRPC6 is highly expressed in the lung, but its role in allergic processes is still poorly understood.

Novel APP/Aβ mutation K16N produces highly toxic heteromeric Aβ oligomers

Here, we describe a novel missense mutation in the amyloid precursor protein (APP) causing a lysine‐to‐asparagine substitution at position 687 (APP770; herein, referred to as K16N according to amyloid‐β (Aβ) numbering) resulting in an early onset dementia with an autosomal dominant inheritance pattern. The K16N mutation is located exactly at the α‐secretase cleavage site and influences both APP and Aβ. First, due to the K16N mutation APP secretion is affected and a higher amount of Aβ peptides is being produced. Second, Aβ peptides carrying the K16N mutation are unique in that the peptide itself is not harmful to neuronal cells. Severe toxicity, however, is evident upon equimolar mixture of wt and mutant peptides, mimicking the heterozygous state of the subject. Furthermore, Aβ42 K16N inhibits fibril formation of Aβ42 wild‐type. Even more, Aβ42 K16N peptides are protected against clearance activity by the major Aβ‐degrading enzyme neprilysin. Thus the mutation characterized here harbours a combination of risk factors that synergistically may contribute to the development of early onset Alzheimer disease.

Vesicular Synaptobrevin/VAMP2 Levels Guarded by AP180 Control Efficient Neurotransmission.

Neurotransmission depends on synaptic vesicle (SV) exocytosis driven by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex formation of vesicular synaptobrevin/VAMP2 (Syb2). Exocytic fusion is followed by endocytic SV membrane retrieval and the high-fidelity reformation of SVs. Syb2 is the most abundant SV protein with 70 copies per SV, yet, one to three Syb2 molecules appear to be sufficient for basal exocytosis. Here we demonstrate that loss of the Syb2-specific endocytic adaptor AP180 causes a moderate activity-dependent reduction of vesicular Syb2 levels, defects in SV reformation, and a corresponding impairment of neurotransmission that lead to excitatory/inhibitory imbalance, epileptic seizures, and premature death. Further reduction of Syb2 levels in AP180(-/-)/Syb2(+/-) mice results in perinatal lethality, whereas Syb2(+/-) mice partially phenocopy loss of AP180, indicating that reduced vesicular Syb2 levels underlie the observed defects in neurotransmission. Thus, a large vesicular Syb2 pool maintained by AP180 is crucial to sustain efficient neurotransmission and SV reformation.

RIM-binding protein 2 regulates release probability by fine-tuning calcium channel localization at murine hippocampal synapses

Significance Highly regulated and precise positioning of Ca2+ channels at the active zone (AZ) controls Ca2+ nanodomains at release sites. Their exact localization affects vesicular release probability (PVR) and is important for proper synaptic transmission during repetitive stimulation. We provide a detailed analysis of synaptic transmission combined with superresolution imaging of the AZ organization in mouse hippocampal synapses lacking Rab-interacting molecule-binding protein 2 (RIM-BP2). By dual- and triple-channel time-gated stimulated emission depletion (gSTED) microscopy, we directly show that RIM-BP2 fine-tunes voltage-gated Ca2+ channel 2.1 (CaV2.1) localization at the AZ. We reveal that RIM-BP2 likely regulates the Ca2+ nanodomain by positioning CaV2.1 channels close to synaptic vesicle release sites. Loss of RIM-BP2 reduces PVR and alters short-term plasticity. The tight spatial coupling of synaptic vesicles and voltage-gated Ca2+ channels (CaVs) ensures efficient action potential-triggered neurotransmitter release from presynaptic active zones (AZs). Rab-interacting molecule-binding proteins (RIM-BPs) interact with Ca2+ channels and via RIM with other components of the release machinery. Although human RIM-BPs have been implicated in autism spectrum disorders, little is known about the role of mammalian RIM-BPs in synaptic transmission. We investigated RIM-BP2–deficient murine hippocampal neurons in cultures and slices. Short-term facilitation is significantly enhanced in both model systems. Detailed analysis in culture revealed a reduction in initial release probability, which presumably underlies the increased short-term facilitation. Superresolution microscopy revealed an impairment in CaV2.1 clustering at AZs, which likely alters Ca2+ nanodomains at release sites and thereby affects release probability. Additional deletion of RIM-BP1 does not exacerbate the phenotype, indicating that RIM-BP2 is the dominating RIM-BP isoform at these synapses.

Homeostatic regulation of NCAM polysialylation is critical for correct synaptic targeting

During development, axonal projections have a remarkable ability to innervate correct dendritic subcompartments of their target neurons and to form regular neuronal circuits. Altered axonal targeting with formation of synapses on inappropriate neurons may result in neurodevelopmental sequelae, leading to psychiatric disorders. Here we show that altering the expression level of the polysialic acid moiety, which is a developmentally regulated, posttranslational modification of the neural cell adhesion molecule NCAM, critically affects correct circuit formation. Using a chemically modified sialic acid precursor (N-propyl-d-mannosamine), we inhibited the polysialyltransferase ST8SiaII, the principal enzyme involved in polysialylation during development, at selected developmental time-points. This treatment altered NCAM polysialylation while NCAM expression was not affected. Altered polysialylation resulted in an aberrant mossy fiber projection that formed glutamatergic terminals on pyramidal neurons of the CA1 region in organotypic slice cultures and in vivo. Electrophysiological recordings revealed that the ectopic terminals on CA1 pyramids were functional and displayed characteristics of mossy fiber synapses. Moreover, ultrastructural examination indicated a “mossy fiber synapse”-like morphology. We thus conclude that homeostatic regulation of the amount of synthesized polysialic acid at specific developmental stages is essential for correct synaptic targeting and circuit formation during hippocampal development.

Activation of metabotropic GABA receptors increases the energy barrier for vesicle fusion

Neurotransmitter release from presynaptic terminals is under the tight control of various metabotropic receptors. We report here that in addition to the regulation of Ca2+ channel activity, metabotropic GABAB receptors (GABABRs) at murine hippocampal glutamatergic synapses utilize an inhibitory pathway that directly targets the synaptic vesicle release machinery. Acute application of the GABABR agonist baclofen rapidly and reversibly inhibits vesicle fusion, which occurs independently of the SNAP-25 C-terminus. Using applications of hypertonic sucrose solutions, we find that the size of the readily releasable pool remains unchanged by GABABR activation, but the sensitivity of primed vesicles to hypertonic stimuli appears lowered as the response amplitudes at intermediate sucrose concentrations are smaller and release kinetics are slowed. These data show that presynaptic GABABRs can inhibit neurotransmitter release directly by increasing the energy barrier for vesicle fusion.

Autaptic cultures of single hippocampal granule cells of mice and rats

When a single neuron is grown on a small island of glial cells, the neuron forms synapses onto itself. The so‐called autaptic culture systems have proven extremely valuable in elucidating basic mechanisms of synaptic transmission, as they allow application of technical approaches that cannot be used in slice preparations. However, this method has been almost exclusively used for pyramidal cells and interneurons. In this study, we generated autaptic cultures from granule cells isolated from the dentate gyrus of rodent hippocampi. Our subsequent morphological and functional characterisation of these cells confirms that this culture model is suitable for investigating basic mechanisms of granule cell synaptic transmission. Importantly, the autosynaptic connectivity allows recordings of pure mossy fibre miniature EPSCs, which are not possible in slice preparations. Further, by fast application of hypertonic sucrose solutions it is possible to directly measure the readily releasable pool and to calculate the probability of vesicular release.

Cannabinoid type 2 receptors mediate a cell type-specific self-inhibition in cortical neurons

ABSTRACT Endogenous cannabinoids are diffusible lipid ligands of the main cannabinoid receptors type 1 and 2 (CB1R and CB2R). In the central nervous system endocannabinoids are produced in an activity‐dependent manner and have been identified as retrograde modulators of synaptic transmission. Additionally, some neurons display a cell‐autonomous slow self‐inhibition (SSI) mediated by endocannabinoids. In these neurons, repetitive action potential firing triggers the production of endocannabinoids, which induce a long‐lasting hyperpolarization of the membrane potential, rendering the cells less excitable. Different endocannabinoid receptors and effector mechanisms have been described underlying SSI in different cell types and brain areas. Here, we investigate SSI in neurons of layer 2/3 in the somatosensory cortex. High‐frequency bursts of action potentials induced SSI in pyramidal cells (PC) and regular spiking non‐pyramidal cells (RSNPC), but not in fast‐spiking interneurons (FS). In RSNPCs the hyperpolarization was accompanied by a change in input resistance due to the activation of G protein‐coupled inward‐rectifying K+ (GIRK) channels. A CB2R‐specific agonist induced the long‐lasting hyperpolarization, whereas preincubation with a CB2R‐specific inverse agonist suppressed SSI. Additionally, using cannabinoid receptor knockout mice, we found that SSI was still intact in CB1R‐deficient but abolished in CB2R‐deficient mice. Taken together, we describe an additional SSI mechanism in which the activity‐induced release of endocannabinoids activates GIRK channels via CB2Rs. These findings expand our knowledge about cell type‐specific differential neuronal cannabinoid receptor signaling and suggest CB2R‐selective compounds as potential therapeutic approaches. HIGHLIGHTSTrains of APs induce a long‐lasting hyperpolarization in pyramidal cells and regular spiking non‐pyramidal cells but not in fastspiking interneurons.SSI is mediated by activation of GIRK channels.Activation of CB2Rs mediate SSI.