Rapid and reversible optogenetic silencing of synaptic transmission by clustering of synaptic vesicles

Flower/FLWR-1 regulates neuronal activity via the plasma membrane Ca2+ ATPase to promote recycling of synaptic vesicles

The Flower protein was suggested to couple the fusion of synaptic vesicles (SVs) to their recycling in different model organisms. It is supposed to trigger activity-dependent bulk endocytosis by conducting Ca2+ at endocytic sites. However, this mode of action is debated. Here, we investigated the role of the Caenorhabditis elegans homologue FLWR-1 in neurotransmission. Our results confirm that FLWR-1 facilitates the recycling of SVs at the neuromuscular junction (NMJ). Ultrastructural analysis of synaptic boutons after hyperstimulation revealed an accumulation of large endocytic structures in flwr-1 mutants. These findings do not support a role of FLWR-1 in the formation of bulk endosomes but rather a function in their breakdown. Unexpectedly, the loss of FLWR-1 led to increased neuronal Ca2+ levels in axon terminals during stimulation, particularly in GABAergic motor neurons, causing excitation-inhibition imbalance. We found that this increased NMJ transmission might be caused by deregulation of MCA-3, the nematode orthologue of the plasma membrane Ca2+ ATPase (PMCA). In vivo molecular interactions indicated that FLWR-1 may be a positive regulator of the PMCA and might influence its recycling through modification of plasma membrane levels of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2).

Loss of neuropeptidergic regulation of cholinergic transmission induces homeostatic compensation in muscle cells to preserve synaptic strength

Chemical synaptic transmission at the neuromuscular junction (NMJ) is regulated by electrical activity of the motor circuit, but may also be affected by neuromodulation. Here, we assessed the role of neuropeptide signaling in the plasticity of NMJ function in Caenorhabditis elegans. We show that the CAPS (Ca2+-dependent activator protein for secretion) ortholog UNC-31, which regulates exocytosis of dense core vesicles, affects both pre- and post-synaptic functional properties, as well as NMJ-mediated locomotion. Despite reduced evoked acetylcholine (ACh) transmission, the loss of unc-31 results in a more vigorous response to presynaptic stimulation, i.e., enhanced muscle contraction and Ca2+ transients. Based on expression profiles, we identified neuropeptides involved in both cholinergic (FLP-6, FLP-15, NLP-9, NLP-15, NLP-21, and NLP-38) and GABAergic motor neurons (FLP-15, NLP-15), that mediate normal transmission at the NMJ. In the absence of these peptides, neurons fail to upregulate their ACh output in response to increased cAMP signaling; for flp-15; nlp-15 double mutants, we observed overall increased postsynaptic currents, indicating that these neuropeptides may be inhibitory. We also identified proprotein convertases encoded by aex-5/kpc-3 and egl-3/kpc-2 that act synergistically to generate these neuropeptides. We propose that postsynaptic homeostatic scaling, mediated by increased muscle activation, likely through excitability, might compensate for the reduced cholinergic transmission in mutants affected for neuropeptide signaling, thus maintaining net synaptic strength. We show that in the absence of UNC-31 muscle excitability is modulated by upregulating the expression of the muscular L-type voltage-gated Ca2+ channel EGL-19. Our results unveil a role for neuropeptidergic regulation in synaptic plasticity, linking changes in presynaptic transmission to compensatory changes in muscle excitability.

Functional interrogation of neuronal connections by chemoptogenetic presynaptic ablation

Most neurons are embedded in multiple circuits, with signaling to distinct postsynaptic partners playing functionally different roles. The function of specific connections can be interrogated using synaptically localized optogenetic effectors, however these tools are often experimentally difficult to validate or produce paradoxical outcomes. We have developed a system for photoablation of synaptic connections originating from genetically defined neurons, based on presynaptic localization of the fluorogen activating protein dL5** that acts as a photosensitizer when bound to a cell-permeable dye. Using the well mapped zebrafish escape circuit as a readout, we first show that cytoplasmically expressed dL5** enables efficient spatially targeted neuronal ablation using near infra-red light. We then demonstrate that spatially patterned illumination of presynaptically localized dL5** can effectively disconnect neurons from selected downstream partners, producing precise behavioral deficits. This technique should be applicable to almost any genetically tractable neuronal circuit, enabling precise manipulation of functional connectivity within the nervous system.

Enriched environment improves memory function by promoting synaptic remodeling in vascular dementia rats

Vascular dementia (VaD), attributed to cerebrovascular pathology, is a leading cause of cognitive decline, characterized by memory loss, bradyphrenia, and affective lability, with memory deficits being particularly pronounced. The potential of enriched environment (EE) to ameliorate cognitive impairments by enhancing hippocampal synaptic plasticity, neurogenesis, and white matter remodeling has garnered considerable interest. In this study, we used a rat model for VaD through the procedure of bilateral common carotid artery ligation (BCCAO). We randomly assigned male Sprague-Dawley (SD) rats to three groups: the control sham-operated group (Sham group), the surgery-induced dementia group (BCCAO group), and the surgery-induced dementia group with enriched environment (EE group). The Sham and BCCAO groups were kept under standard lab conditions, whereas the EE group was housed in an enriched setting. Employing a behavioral assay battery, we observed that EE intervention significantly improved the spatial learning and memory performance in the Morris water maze. Subsequent neuromorphological assessments utilizing transmission electron microscopy disclosed an increase in synaptic density and postsynaptic density (PSD) thickness within the hippocampal CA1 region, indicative of structural synaptic modulation. Further probing into the molecular underpinnings revealed that EE upregulated the expression of PSD95, corroborating its role in enhancing cognitive faculties. Additionally, our investigation into the PGC-1α/FNDC5/BDNF pathway demonstrated that EE intervention elevated the expression of these neurotrophic factors, suggesting a mechanistic link to synaptic and cognitive restoration. In summation, our findings elucidate the neurorestorative potential of EE in a preclinical VaD model, presenting a non-pharmacological intervention that modulates synaptic architecture and activates neuroprotective pathways. The observed correlations between synaptic remodeling and cognitive enhancement underscore the therapeutic relevance of EE in VaD, warranting further investigation for clinical applications.

Tools and methods for cell ablation and cell inhibition in Caenorhabditis elegans

To understand the function of cells such as neurons within an organism, it can be instrumental to inhibit cellular function, or to remove the cell (type) from the organism, and thus to observe the consequences on organismic and/or circuit function and animal behavior. A range of approaches and tools were developed and used over the past few decades that act either constitutively or acutely and reversibly, in systemic or local fashion. These approaches make use of either drugs or genetically encoded tools. Also, there are acutely acting inhibitory tools that require an exogenous trigger like light. Here, we give an overview of such methods developed and used in the nematode Caenorhabditis elegans.

Precise Control of Intracellular Trafficking and Receptor-Mediated Endocytosis in Living Cells and Behaving Animals

Intracellular trafficking, an extremely complex network, dynamically orchestrates nearly all cellular activities. A versatile method that enables the manipulation of target transport pathways with high spatiotemporal accuracy in vitro and in vivo is required to study how this network coordinates its functions. Here, a new method called RIVET (Rapid Immobilization of target Vesicles on Engaged Tracks) is presented. Utilizing inducible dimerization between target vesicles and selective cytoskeletons, RIVET can spatiotemporally halt numerous intracellular trafficking pathways within seconds in a reversible manner. Its highly specific perturbations allow for the real-time dissection of the dynamic relationships among different trafficking pathways. Moreover, RIVET is capable of inhibiting receptor-mediated endocytosis. This versatile system can be applied from the cellular level to whole organisms. RIVET opens up new avenues for studying intracellular trafficking under various physiological and pathological conditions and offers potential strategies for treating trafficking-related disorders.

Precision in situ cryogenic correlative light and electron microscopy of optogenetically positioned organelles

Unambiguous targeting of cellular structures for in situ cryo-electron microscopy in the heterogeneous, dense and compacted environment of the cytoplasm remains challenging. Here, we have developed a cryogenic correlative light and electron microscopy (cryo-CLEM) workflow that utilizes thin cells grown on a mechanically defined substratum for rapid analysis of organelles and macromolecular complexes by cryo-electron tomography (cryo-ET). We coupled these advancements with optogenetics to redistribute perinuclear-localised organelles to the cell periphery, allowing visualisation of organelles that would otherwise be positioned in cellular regions too thick for cryo-ET. This reliable and robust workflow allows for fast in situ analyses without the requirement for cryo-focused ion beam milling. Using this protocol, cells can be frozen, imaged by cryo-fluorescence microscopy and be ready for batch cryo-ET within a day.

The synaptic vesicle cluster as a controller of pre- and postsynaptic structure and function

The synaptic vesicle cluster (SVC) is an essential component of chemical synapses, which provides neurotransmitter-loaded vesicles during synaptic activity, at the same time as also controlling the local concentrations of numerous exo- and endocytosis cofactors. In addition, the SVC hosts molecules that participate in other aspects of synaptic function, from cytoskeletal components to adhesion proteins, and affects the location and function of organelles such as mitochondria and the endoplasmic reticulum. We argue here that these features extend the functional involvement of the SVC in synapse formation, signalling and plasticity, as well as synapse stabilization and metabolism. We also propose that changes in the size of the SVC coalesce with changes in the postsynaptic compartment, supporting the interplay between pre- and postsynaptic dynamics. Thereby, the SVC could be seen as an 'all-in-one' regulator of synaptic structure and function, which should be investigated in more detail, to reveal molecular mechanisms that control synaptic function and heterogeneity.

Advances in the labelling and selective manipulation of synapses

Synapses are highly specialized neuronal structures that are essential for neurotransmission, and they are dynamically regulated throughout the lifetime. Although accumulating evidence indicates that these structures are crucial for information processing and storage in the brain, their precise roles beyond neurotransmission are yet to be fully appreciated. Genetically encoded fluorescent tools have deepened our understanding of synaptic structure and function, but developing an ideal methodology to selectively visualize, label and manipulate synapses remains challenging. Here, we provide an overview of currently available synapse labelling techniques and describe their extension to enable synapse manipulation. We categorize these approaches on the basis of their conceptual bases and target molecules, compare their advantages and limitations and propose potential modifications to improve their effectiveness. These methods have broad utility, particularly for investigating mechanisms of synaptic function and synaptopathy.

RIM and RIM-Binding Protein Localize Synaptic CaV2 Channels to Differentially Regulate Transmission in Neuronal Circuits

At chemical synapses, voltage-gated Ca2+ channels (VGCCs) translate electrical signals into a trigger for synaptic vesicle (SV) fusion. VGCCs and the Ca2+ microdomains they elicit must be located precisely to primed SVs to evoke rapid transmitter release. Localization is mediated by Rab3-interacting molecule (RIM) and RIM-binding proteins, which interact and bind to the C terminus of the CaV2 VGCC α-subunit. We studied this machinery at the mixed cholinergic/GABAergic neuromuscular junction of Caenorhabditis elegans hermaphrodites. rimb-1 mutants had mild synaptic defects, through loosening the anchoring of UNC-2/CaV2 and delaying the onset of SV fusion. UNC-10/RIM deletion much more severely affected transmission. Although postsynaptic depolarization was reduced, rimb-1 mutants had increased cholinergic (but reduced GABAergic) transmission, to compensate for the delayed release. This did not occur when the excitation-inhibition (E-I) balance was altered by removing GABA transmission. Further analyses of GABA defective mutants and GABAA or GABAB receptor deletions, as well as cholinergic rescue of RIMB-1, emphasized that GABA neurons may be more affected than cholinergic neurons. Thus, RIMB-1 function differentially affects excitation-inhibition balance in the different motor neurons, and RIMB-1 thus may differentially regulate transmission within circuits. Untethering the UNC-2/CaV2 channel by removing its C-terminal PDZ ligand exacerbated the rimb-1 defects, and similar phenotypes resulted from acute degradation of the CaV2 β-subunit CCB-1. Therefore, untethering of the CaV2 complex is as severe as its elimination, yet it does not abolish transmission, likely due to compensation by CaV1. Thus, robustness and flexibility of synaptic transmission emerge from VGCC regulation.

A bistable inhibitory optoGPCR for multiplexed optogenetic control of neural circuits

Information is transmitted between brain regions through the release of neurotransmitters from long-range projecting axons. Understanding how the activity of such long-range connections contributes to behavior requires efficient methods for reversibly manipulating their function. Chemogenetic and optogenetic tools, acting through endogenous G-protein-coupled receptor pathways, can be used to modulate synaptic transmission, but existing tools are limited in sensitivity, spatiotemporal precision or spectral multiplexing capabilities. Here we systematically evaluated multiple bistable opsins for optogenetic applications and found that the Platynereis dumerilii ciliary opsin (PdCO) is an efficient, versatile, light-activated bistable G-protein-coupled receptor that can suppress synaptic transmission in mammalian neurons with high temporal precision in vivo. PdCO has useful biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. We demonstrate that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping.

Closed-loop experiments and brain machine interfaces with multiphoton microscopy

In the field of neuroscience, the importance of constructing closed-loop experimental systems has increased in conjunction with technological advances in measuring and controlling neural activity in live animals. We provide an overview of recent technological advances in the field, focusing on closed-loop experimental systems where multiphoton microscopy-the only method capable of recording and controlling targeted population activity of neurons at a single-cell resolution in vivo-works through real-time feedback. Specifically, we present some examples of brain machine interfaces (BMIs) using in vivo two-photon calcium imaging and discuss applications of two-photon optogenetic stimulation and adaptive optics to real-time BMIs. We also consider conditions for realizing future optical BMIs at the synaptic level, and their possible roles in understanding the computational principles of the brain.

A bistable inhibitory OptoGPCR for multiplexed optogenetic control of neural circuits

Information is transmitted between brain regions through the release of neurotransmitters from long-range projecting axons. Understanding how the activity of such long-range connections contributes to behavior requires efficient methods for reversibly manipulating their function. Chemogenetic and optogenetic tools, acting through endogenous G-protein coupled receptor (GPCRs) pathways, can be used to modulate synaptic transmission, but existing tools are limited in sensitivity, spatiotemporal precision, or spectral multiplexing capabilities. Here we systematically evaluated multiple bistable opsins for optogenetic applications and found that the Platynereis dumerilii ciliary opsin (PdCO) is an efficient, versatile, light-activated bistable GPCR that can suppress synaptic transmission in mammalian neurons with high temporal precision in-vivo. PdCO has superior biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. We demonstrate that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping.

pOpsicle: An all-optical reporter system for synaptic vesicle recycling combining pH-sensitive fluorescent proteins with optogenetic manipulation of neuronal activity

pH-sensitive fluorescent proteins are widely used to study synaptic vesicle (SV) fusion and recycling. When targeted to the lumen of SVs, fluorescence of these proteins is quenched by the acidic pH. Following SV fusion, they are exposed to extracellular neutral pH, resulting in a fluorescence increase. SV fusion, recycling and acidification can thus be tracked by tagging integral SV proteins with pH-sensitive proteins. Neurotransmission is generally activated by electrical stimulation, which is not feasible in small, intact animals. Previous in vivo approaches depended on distinct (sensory) stimuli, thus limiting the addressable neuron types. To overcome these limitations, we established an all-optical approach to stimulate and visualize SV fusion and recycling. We combined distinct pH-sensitive fluorescent proteins (inserted into the SV protein synaptogyrin) and light-gated channelrhodopsins (ChRs) for optical stimulation, overcoming optical crosstalk and thus enabling an all-optical approach. We generated two different variants of the pH-sensitive optogenetic reporter of vesicle recycling (pOpsicle) and tested them in cholinergic neurons of intact Caenorhabditis elegans nematodes. First, we combined the red fluorescent protein pHuji with the blue-light gated ChR2(H134R), and second, the green fluorescent pHluorin combined with the novel red-shifted ChR ChrimsonSA. In both cases, fluorescence increases were observed after optical stimulation. Increase and subsequent decline of fluorescence was affected by mutations of proteins involved in SV fusion and endocytosis. These results establish pOpsicle as a non-invasive, all-optical approach to investigate different steps of the SV cycle.