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Live fluorescent imaging of murine hippocampal slices shows that NMDAR-dependent glutamate signalling leads to postsynaptic BDNF release, with associated signalling of its receptor, TrkB, on the same dendritic spine, suggesting autocrine BDNF signalling. Mechanisms of neuronal plasticity Secreted messenger molecules such as brain-derived neurotrophic factor (BDNF) are known to participate in various forms of neuronal plasticity, such as long-term synaptic potentiation (LTP) and associated changes in dendritic spine morphology, but the exact sites of BDNF release and action remain poorly defined. Two papers from Ryohei Yasuda's lab, published in this issue of Nature , tackle this question. Stephen Harward et al . use live fluorescent imaging of murine hippocampal slices to show that NMDAR-dependent glutamate signalling leads to postsynaptic BDNF release, with associated signalling of its receptor, TrkB, on the same dendritic spine, suggesting autocrine BDNF signalling. In the second study Nathan Hedrick et al . find that the three small GTPases Rac1, RhoA and Cdc42 are differentially involved in structural long-term potentiation of rodent dendritic spines, simultaneously ensuring signal specificity while also priming the system for plasticity. Taken together these results suggest molecular mechanisms for both signal specificity at single spines and synaptic cross-talk, a unique biochemical computation involved in neuronal plasticity and learning. Brain-derived neurotrophic factor (BDNF) and its receptor TrkB are crucial for many forms of neuronal plasticity 1 , 2 , 3 , 4 , 5 , 6 , including structural long-term potentiation (sLTP) 7 , 8 , which is a correlate of an animal’s learning 7 , 9 , 10 , 11 , 12 . However, it is unknown whether BDNF release and TrkB activation occur during sLTP, and if so, when and where. Here, using a fluorescence resonance energy transfer-based sensor for TrkB and two-photon fluorescence lifetime imaging microscopy 13 , 14 , 15 , 16 , we monitor TrkB activity in single dendritic spines of CA1 pyramidal neurons in cultured murine hippocampal slices. In response to sLTP induction 9 , 14 , 15 , 16 , we find fast (onset < 1 min) and sustained (>20 min) activation of TrkB in the stimulated spine that depends on NMDAR ( N -methyl- d -aspartate receptor) and CaMKII signalling and on postsynaptically synthesized BDNF. We confirm the presence of postsynaptic BDNF using electron microscopy to localize endogenous BDNF to dendrites and spines of hippocampal CA1 pyramidal neurons. Consistent with these findings, we also show rapid, glutamate-uncaging-evoked, time-locked BDNF release from single dendritic spines using BDNF fused to superecliptic pHluorin 17 , 18 , 19 . We demonstrate that this postsynaptic BDNF–TrkB signalling pathway is necessary for both structural and functional LTP 20 . Together, these findings reveal a spine-autonomous, autocrine signalling mechanism involving NMDAR–CaMKII-dependent BDNF release from stimulated dendritic spines and subsequent TrkB activation on these same spines that is crucial for structural and functional plasticity.

Recent work has highlighted roles for thermodynamic phase behavior in diverse cellular processes. Proteins and nucleic acids can phase separate into three-dimensional liquid droplets in the cytoplasm and nucleus and the plasma membrane of animal cells appears tuned close to a two-dimensional liquid–liquid critical point. In some examples, cytoplasmic proteins aggregate at plasma membrane domains, forming structures such as the postsynaptic density and diverse signaling clusters. Here we examine the physics of these surface densities, employing minimal simulations of polymers prone to phase separation coupled to an Ising membrane surface in conjunction with a complementary Landau theory. We argue that these surface densities are a phase reminiscent of prewetting, in which a molecularly thin three-dimensional liquid forms on a usually solid surface. However, in surface densities the solid surface is replaced by a membrane with an independent propensity to phase separate. We show that proximity to criticality in the membrane dramatically increases the parameter regime in which a prewetting-like transition occurs, leading to a broad region where coexisting surface phases can form even when a bulk phase is unstable. Our simulations naturally exhibit three-surface phase coexistence even though both the membrane and the polymer bulk only display two-phase coexistence on their own. We argue that the physics of these surface densities may be shared with diverse functional structures seen in eukaryotic cells.

Dendritic spines, the postsynaptic compartments of excitatory neurotransmission, have different shapes classified from ‘stubby’ to ‘mushroom-like’. Whereas mushroom spines are essential for adult brain function, stubby spines disappear during brain maturation. It is still unclear whether and how they differ in protein composition. To address this, we combined electron microscopy and quantitative biochemistry with super-resolution microscopy to annotate more than 47,000 spines for more than 100 synaptic targets. Surprisingly, mushroom and stubby spines have similar average protein copy numbers and topologies. However, an analysis of the correlation of each protein to the postsynaptic density mass, used as a marker of synaptic strength, showed substantially more significant results for the mushroom spines. Secretion and trafficking proteins correlated particularly poorly to the strength of stubby spines. This suggests that stubby spines are less likely to adequately respond to dynamic changes in synaptic transmission than mushroom spines, which possibly explains their loss during brain maturation. This work provides a first molecular view of dendritic spines, for both the mushroom and stubby classes, obtained by integrating electron microscopy, quantitative biochemistry, super-resolution microscopy and 3D molecular visualizations.

Re-expression of the Shank3 gene in adult mice results in improvements in synaptic protein composition and spine density in the striatum; Shank3 also rescues autism-like features such as social interaction and grooming behaviour, and the results suggest that aspects of autism spectrum disorders may be reversible in adulthood. Autism-like signs reversed by Shank3 Mutations in the Shank3 gene have been linked to autism, and mice lacking Shank3 expression display features of autism, including social deficits, anxiety and repetitive behaviour, as well as defects in striatal synapses. Guoping Feng and colleagues now show that re-expression of Shank3 in adult mice reversed the synaptic changes and increased spine density in the striatum. It also selectively rescued social interaction and grooming behaviour — two core features of autism — whereas anxiety and motor impairments could only be prevented by Shank3 re-expression during development. These findings show that Shank3 expression can affect neural function post-development, and suggest that aspects of autism spectrum disorder pathology may be reversible in adulthood. Because autism spectrum disorders are neurodevelopmental disorders and patients typically display symptoms before the age of three 1 , one of the key questions in autism research is whether the pathology is reversible in adults. Here we investigate the developmental requirement of Shank3 in mice, a prominent monogenic autism gene that is estimated to contribute to approximately 1% of all autism spectrum disorder cases 2 , 3 , 4 , 5 , 6 . SHANK3 is a postsynaptic scaffold protein that regulates synaptic development, function and plasticity by orchestrating the assembly of postsynaptic density macromolecular signalling complex 7 , 8 , 9 . Disruptions of the Shank3 gene in mouse models have resulted in synaptic defects and autistic-like behaviours including anxiety, social interaction deficits, and repetitive behaviour 10 , 11 , 12 , 13 . We generated a novel Shank3 conditional knock-in mouse model, and show that re-expression of the Shank3 gene in adult mice led to improvements in synaptic protein composition, spine density and neural function in the striatum. We also provide behavioural evidence that certain behavioural abnormalities including social interaction deficit and repetitive grooming behaviour could be rescued, while anxiety and motor coordination deficit could not be recovered in adulthood. Together, these results reveal the profound effect of post-developmental activation of Shank3 expression on neural function, and demonstrate a certain degree of continued plasticity in the adult diseased brain.

Inhibitory synapses dampen neuronal activity through postsynaptic hyperpolarization.The composition of the inhibitory postsynapse and the mechanistic basis of its regulation, however, remain poorly understood. We used an in vivo chemico-genetic proximity-labeling approach to discover inhibitory postsynaptic proteins. Quantitative mass spectrometry not only recapitulated known inhibitory postsynaptic proteins but also revealed a large network of new proteins, many of which are either implicated in neurodevelopmental disorders or are of unknown function. Clustered regularly interspaced short palindromic repeats (CRISPR) depletion of one of these previously uncharacterized proteins, InSyn1, led to decreased postsynaptic inhibitory sites, reduced the frequency of miniature inhibitory currents, and increased excitability in the hippocampus. Our findings uncover a rich and functionally diverse assemblage of previously unknown proteins that regulate postsynaptic inhibition and might contribute to developmental brain disorders.

Molecular signaling mechanisms underlying Alzheimer's disease (AD) remain unclear. Maintenance of memory and synaptic plasticity depend on de novo protein synthesis, dysregulation of which is implicated in AD. Recent studies showed AD-associated hyperphosphorylation of mRNA translation factor eukaryotic elongation factor 2 (eEF2), which results in inhibition of protein synthesis. We tested to determine whether suppression of eEF2 phosphorylation could improve protein synthesis capacity and AD-associated cognitive and synaptic impairments. Genetic reduction of the eEF2 kinase (eEF2K) in 2 AD mouse models suppressed AD-associated eEF2 hyperphosphorylation and improved memory deficits and hippocampal long-term potentiation (LTP) impairments without altering brain amyloid β (Aβ) pathology. Furthermore, eEF2K reduction alleviated AD-associated defects in dendritic spine morphology, postsynaptic density formation, de novo protein synthesis, and dendritic polyribosome assembly. Our results link eEF2K/eEF2 signaling dysregulation to AD pathophysiology and therefore offer a feasible therapeutic target.

The three small GTPases Rac1, RhoA and Cdc42 are differentially involved in structural long-term potentiation of rodent dendritic spines, simultaneously ensuring signal specificity and also priming the system for plasticity. Mechanisms of neuronal plasticity Secreted messenger molecules such as brain-derived neurotrophic factor (BDNF) are known to participate in various forms of neuronal plasticity, such as long-term synaptic potentiation (LTP) and associated changes in dendritic spine morphology, but the exact sites of BDNF release and action remain poorly defined. Two papers from Ryohei Yasuda's lab, published in this issue of Nature , tackle this question. Stephen Harward et al . use live fluorescent imaging of murine hippocampal slices to show that NMDAR-dependent glutamate signalling leads to postsynaptic BDNF release, with associated signalling of its receptor, TrkB, on the same dendritic spine, suggesting autocrine BDNF signalling. In the second study Nathan Hedrick et al . find that the three small GTPases Rac1, RhoA and Cdc42 are differentially involved in structural long-term potentiation of rodent dendritic spines, simultaneously ensuring signal specificity while also priming the system for plasticity. Taken together these results suggest molecular mechanisms for both signal specificity at single spines and synaptic cross-talk, a unique biochemical computation involved in neuronal plasticity and learning. The Rho GTPase proteins Rac1, RhoA and Cdc42 have a central role in regulating the actin cytoskeleton in dendritic spines 1 , thereby exerting control over the structural and functional plasticity of spines 2 , 3 , 4 , 5 and, ultimately, learning and memory 6 , 7 , 8 . Although previous work has shown that precise spatiotemporal coordination of these GTPases is crucial for some forms of cell morphogenesis 9 , the nature of such coordination during structural spine plasticity is unclear. Here we describe a three-molecule model of structural long-term potentiation (sLTP) of murine dendritic spines, implicating the localized, coincident activation of Rac1, RhoA and Cdc42 as a causal signal of sLTP. This model posits that complete tripartite signal overlap in spines confers sLTP, but that partial overlap primes spines for structural plasticity. By monitoring the spatiotemporal activation patterns of these GTPases during sLTP, we find that such spatiotemporal signal complementation simultaneously explains three integral features of plasticity: the facilitation of plasticity by brain-derived neurotrophic factor (BDNF), the postsynaptic source of which activates Cdc42 and Rac1, but not RhoA; heterosynaptic facilitation of sLTP, which is conveyed by diffusive Rac1 and RhoA activity; and input specificity, which is afforded by spine-restricted Cdc42 activity. Thus, we present a form of biochemical computation in dendrites involving the controlled complementation of three molecules that simultaneously ensures signal specificity and primes the system for plasticity.

Synaptic vesicle fusion, as evoked by action potentials, is confined to presynaptic protein nanoclusters, which are closely aligned with concentrated postsynaptic receptors and their scaffolding proteins—an organization termed a ‘nanocolumn’. Molecular organization in a single synapse Efficient neurotransmission has long been suspected to require precise alignment between pre-synaptic vesicle release sites and post-synaptic receptors, but direct observations have been hampered by the physics of light-microscopy. Thomas Blanpied and colleagues use hyper-resolution microscopy — which overcomes the diffraction barrier — to reveal that vesicular fusion at single synapses, as evoked by action potentials, is confined to pre-synaptic protein nanoclusters. The nanoclusters are closely aligned with concentrated post-synaptic receptors and their scaffolding proteins. The resulting molecular 'nanocolumns' are reorganized during NMDA receptor-dependent plasticity, and the authors suggest that they may contribute to the maintenance and regulation of synaptic efficiency. Synaptic transmission is maintained by a delicate, sub-synaptic molecular architecture, and even mild alterations in synapse structure drive functional changes during experience-dependent plasticity and pathological disorders 1 , 2 . Key to this architecture is how the distribution of presynaptic vesicle fusion sites corresponds to the position of receptors in the postsynaptic density. However, while it has long been recognized that this spatial relationship modulates synaptic strength 3 , it has not been precisely described, owing in part to the limited resolution of light microscopy. Using localization microscopy, here we show that key proteins mediating vesicle priming and fusion are mutually co-enriched within nanometre-scale subregions of the presynaptic active zone. Through development of a new method to map vesicle fusion positions within single synapses in cultured rat hippocampal neurons, we find that action-potential-evoked fusion is guided by this protein gradient and occurs preferentially in confined areas with higher local density of Rab3-interacting molecule (RIM) within the active zones. These presynaptic RIM nanoclusters closely align with concentrated postsynaptic receptors and scaffolding proteins 4 , 5 , 6 , suggesting the existence of a trans-synaptic molecular ‘nanocolumn’. Thus, we propose that the nanoarchitecture of the active zone directs action-potential-evoked vesicle fusion to occur preferentially at sites directly opposing postsynaptic receptor–scaffold ensembles. Remarkably, NMDA receptor activation triggered distinct phases of plasticity in which postsynaptic reorganization was followed by trans-synaptic nanoscale realignment. This architecture suggests a simple organizational principle of central nervous system synapses to maintain and modulate synaptic efficiency.

Inhibitory synapses are also known as symmetric synapses due to their lack of prominent postsynaptic densities (PSDs) under a conventional electron microscope (EM). Recent cryo-EM tomography studies indicated that inhibitory synapses also contain PSDs, albeit with a rather thin sheet-like structure. It is not known how such inhibitory PSD (iPSD) sheet might form. Here, we demonstrate that the key inhibitory synapse scaffold protein gephyrin, when in complex with either glycine or GABA A receptors, spontaneously forms highly condensed molecular assemblies via phase separation both in solution and on supported membrane bilayers. Multivalent and specific interactions between the dimeric E-domain of gephyrin and the glycine/GABA A receptor multimer are essential for the iPSD condensate formation. Gephyrin alone does not form condensates. The linker between the G- and E-domains of gephyrin inhibits the iPSD condensate formation via autoinhibition. Phosphorylation of specific residues in the linker or binding of target proteins such as dynein light chain to the linker domain regulates gephyrin-mediated glycine/GABA A receptor clustering. Thus, analogous to excitatory PSDs, iPSDs are also formed by phase separation-mediated condensation of scaffold protein/neurotransmitter receptor complexes.

The postsynaptic density (PSD) is a massive protein complex, critical for synaptic strength and plasticity in excitatory neurons. Here, the scaffolding protein PSD-95 plays a crucial role as it organizes key PSD components essential for synaptic signaling, development, and survival. Recently, variants in DLG4 encoding PSD-95 were found to cause a neurodevelopmental disorder with a variety of clinical features including intellectual disability, developmental delay, and epilepsy. Genetic variants in several of the interaction partners of PSD-95 are associated with similar phenotypes, suggesting that deficient PSD-95 may affect the interaction partners, explaining the overlapping symptoms. Here, we review the transmembrane interaction partners of PSD-95 and their association with neurodevelopmental disorders. We assess how the structural changes induced by DLG4 missense variants may disrupt or alter such protein–protein interactions, and we argue that the pathological effect of DLG4 variants is, at least partly, exerted indirectly through interaction partners of PSD-95. This review presents a direction for functional studies to elucidate the pathogenic mechanism of deficient PSD-95, providing clues for therapeutic strategies.