Nanoscale segregation of actin nucleation and elongation factors determines dendritic spine protrusion

The Nebulin Family LIM and SH3 Proteins Regulate Postsynaptic Development and Function

Neuronal dendrites have specialized actin-rich structures called dendritic spines that receive and integrate most excitatory synaptic inputs. The stabilization of dendrites and spines during neuronal maturation is essential for proper neural circuit formation. Changes in dendritic morphology and stability are largely mediated by regulation of the actin cytoskeleton; however, the underlying mechanisms remain to be fully elucidated. Here, we present evidence that the nebulin family members LASP1 and LASP2 play an important role in the postsynaptic development of rat hippocampal neurons from both sexes. We find that both LASP1 and LASP2 are enriched in dendritic spines, and their knockdown impairs spine development and synapse formation. Furthermore, LASP2 exerts a distinct role in dendritic arbor and dendritic spine stabilization. Importantly, the actin-binding N-terminal LIM domain and nebulin repeats of LASP2 are required for spine stability and dendritic arbor complexity. These findings identify LASP1 and LASP2 as novel regulators of neuronal circuitry.SIGNIFICANCE STATEMENT Proper regulation of the actin cytoskeleton is essential for the structural stability of dendrites and dendritic spines. Consequently, the malformation of dendritic structures accompanies numerous neurologic disorders, such as schizophrenia and autism. Nebulin family members are best known for their role in regulating the stabilization and function of actin thin filaments in muscle. The two smallest family members, LASP1 and LASP2, are more structurally diverse and are expressed in a broader array of tissues. While both LASP1 and LASP2 are highly expressed in the brain, little is currently known about their function in the nervous system. In this study, we demonstrate the first evidence that LASP1 and LASP2 are involved in the formation and long-term maintenance of dendrites and dendritic spines.

Myosin XVI Regulates Actin Cytoskeleton Dynamics in Dendritic Spines of Purkinje Cells and Affects Presynaptic Organization

The actin cytoskeleton is crucial for function and morphology of neuronal synapses. Moreover, altered regulation of the neuronal actin cytoskeleton has been implicated in neuropsychiatric diseases such as autism spectrum disorder (ASD). Myosin XVI is a neuronally expressed unconventional myosin known to bind the WAVE regulatory complex (WRC), a regulator of filamentous actin (F-actin) polymerization. Notably, the gene encoding the myosin’s heavy chain (MYO16) shows genetic association with neuropsychiatric disorders including ASD. Here, we investigated whether myosin XVI plays a role for actin cytoskeleton regulation in the dendritic spines of cerebellar Purkinje cells (PCs), a neuronal cell type crucial for motor learning, social cognition and vocalization. We provide evidence that both myosin XVI and the WRC component WAVE1 localize to PC spines. Fluorescence recovery after photobleaching (FRAP) analysis of GFP-actin in cultured PCs shows that Myo16 knockout as well as PC-specific Myo16 knockdown, lead to faster F-actin turnover in the dendritic spines of PCs. We also detect accelerated F-actin turnover upon interference with the WRC, and upon inhibition of Arp2/3 that drives formation of branched F-actin downstream of the WRC. In contrast, inhibition of formins that are responsible for polymerization of linear actin filaments does not cause faster F-actin turnover. Together, our data establish myosin XVI as a regulator of the postsynaptic actin cytoskeleton and suggest that it is an upstream activator of the WRC-Arp2/3 pathway in PC spines. Furthermore, ultra-structural and electrophysiological analyses of Myo16 knockout cerebellum reveals the presence of reduced numbers of synaptic vesicles at presynaptic terminals in the absence of the myosin. Therefore, we here define myosin XVI as an F-actin regulator important for presynaptic organization in the cerebellum.

Characterization of Nanoscale Organization of F-Actin in Morphologically Distinct Dendritic Spines In Vitro Using Supervised Learning

The cytoarchitecture of a neuron is very important in defining morphology and ultrastructure. Although there is a wealth of information on the molecular components that make and regulate these ultrastructures, there is a dearth of understanding of how these changes occur or how they affect neurons in health and disease. Recent advances in nanoscale imaging which resolve cellular structures at the scale of tens of nanometers below the limit of diffraction enable us to understand these structures in fine detail. However, automated analysis of these images is still in its infancy. Towards this goal, attempts have been made to automate the detection and analysis of the cytoskeletal organization of microtubules. To date, evaluation of the nanoscale organization of filamentous actin (F-actin) in neuronal compartments remains challenging. Here, we present an objective paradigm for analysis which adopts supervised learning of nanoscale images of F-actin network in excitatory synapses, obtained by single molecule based super-resolution light microscopy. We have used the proposed analysis to understand the heterogeneity in the organization of F-actin in dendritic spines of primary neuronal cultures from rodents. Our results were validated using ultrastructural data obtained from platinum replica electron microscopy (PREM). The automated analysis approach was used to differentiate the heterogeneity in the nanoscale organization of F-actin in primary neuronal cultures from wild-type (WT) and a transgenic mouse model of Alzheimer’s disease (APP_Swe/PS1ΔE9).

A tessellation-based colocalization analysis approach for single-molecule localization microscopy

Multicolor single-molecule localization microscopy (λSMLM) is a powerful technique to reveal the relative nanoscale organization and potential colocalization between different molecular species. While several standard analysis methods exist for pixel-based images, λSMLM still lacks such a standard. Moreover, existing methods only work on 2D data and are usually sensitive to the relative molecular organization, a very important parameter to consider in quantitative SMLM. Here, we present an efficient, parameter-free colocalization analysis method for 2D and 3D λSMLM using tessellation analysis. We demonstrate that our method allows for the efficient computation of several popular colocalization estimators directly from molecular coordinates and illustrate its capability to analyze multicolor SMLM data in a robust and efficient manner.

Multicolour single-molecule localization microscopy lacks a standard analysis method. Here Levet et al. introduce Coloc-Tesseler, a parameter-free colocalisation analysis method based on tessellation analysis for the efficient analysis of multicolour SMLM data.

Computational geometry analysis of dendritic spines by structured illumination microscopy

Dendritic spines are the postsynaptic sites that receive most of the excitatory synaptic inputs, and thus provide the structural basis for synaptic function. Here, we describe an accurate method for measurement and analysis of spine morphology based on structured illumination microscopy (SIM) and computational geometry in cultured neurons. Surface mesh data converted from SIM images were comparable to data reconstructed from electron microscopic images. Dimensional reduction and machine learning applied to large data sets enabled identification of spine phenotypes caused by genetic mutations in key signal transduction molecules. This method, combined with time-lapse live imaging and glutamate uncaging, could detect plasticity-related changes in spine head curvature. The results suggested that the concave surfaces of spines are important for the long-term structural stabilization of spines by synaptic adhesion molecules.

Mechanism of IRSp53 inhibition by 14-3-3

Filopodia are precursors of dendritic spines and polarized cell migration. The I-BAR-domain protein IRSp53 is a key regulator of filopodia dynamics that couples Rho-GTPase signaling to cytoskeleton and membrane remodeling, playing essential roles in neuronal development and cell motility. Here, we describe the structural-functional basis for 14-3-3-dependent inhibition of IRSp53. Phosphoproteomics, quantitative binding and crystallographic studies demonstrate that 14-3-3 binds to two pairs of phosphorylation sites in IRSp53. Using bicistronic expression, we obtain an IRSp53 heterodimer in which only one subunit is phosphorylated, and show that each subunit of IRSp53 independently binds one 14-3-3 dimer. A FRET-sensor assay using natively phosphorylated IRSp53 reveals opposite conformational changes upon binding of activatory (Cdc42, Eps8) or inhibitory (14-3-3) inputs. Finally, we show that 14-3-3 inhibits IRSp53 binding to membranes. Collectively, our findings support a mechanism whereby phosphorylation-dependent inhibition of IRSp53 by 14-3-3 counters membrane binding and interactions with Cdc42 and downstream cytoskeletal effectors.

IRSp53 is a key regulator of filopodia formation and cell migration. Here, the authors elucidate a mechanism of phosphorylation-dependent inhibition of IRSp53 by 14-3-3, which impedes the interactions of IRSp53 with membranes and downstream cytoskeletal effectors.

Neuronal drebrin A directly interacts with mDia2 formin to inhibit actin assembly

Dendritic spines (DS) are actin-rich postsynaptic terminals of neurons that are critical for higher-order brain functions. Maturation of DS is accompanied by a change in actin architecture from linear to branched filamentous structures. Presumably, the underlying cause of this is a switch in a mode of actin assembly from formin-driven to Arp2/3-mediated via an undefined mechanism. Here we present data suggesting that neuron-specific actin-binding drebrin A may be a part of such a switch. It is well documented that DS are highly enriched in drebrin A, which is critical for their plasticity and function. At the same time, mDia2 is known to mediate the formation of filopodia-type (immature) spines. We found that neuronal drebrin A directly interacts with mDia2 formin. Drebrin inhibits formin-mediated nucleation of actin and abolishes mDia2-induced actin bundling. Using truncated protein constructs we identified the domain requirements for drebrin–mDia2 interaction. We hypothesize that accumulation of drebrin A in DS (that coincides with spine maturation) leads to inhibition of mDia2-driven actin polymerization and, therefore, may contribute to a change in actin architecture from linear to branched filaments.

The Emerging Role of Mechanics in Synapse Formation and Plasticity

The regulation of synaptic strength forms the basis of learning and memory, and is a key factor in understanding neuropathological processes that lead to cognitive decline and dementia. While the mechanical aspects of neuronal development, particularly during axon growth and guidance, have been extensively studied, relatively little is known about the mechanical aspects of synapse formation and plasticity. It is established that a filamentous actin network with complex spatiotemporal behavior controls the dendritic spine shape and size, which is thought to be crucial for activity-dependent synapse plasticity. Accordingly, a number of actin binding proteins have been identified as regulators of synapse plasticity. On the other hand, a number of cell adhesion molecules (CAMs) are found in synapses, some of which form transsynaptic bonds to align the presynaptic active zone (PAZ) with the postsynaptic density (PSD). Considering that these CAMs are key components of cellular mechanotransduction, two critical questions emerge: (i) are synapses mechanically regulated? and (ii) does disrupting the transsynaptic force balance lead to (or exacerbate) synaptic failure? In this mini review article, I will highlight the mechanical aspects of synaptic structures-focusing mainly on cytoskeletal dynamics and CAMs-and discuss potential mechanoregulation of synapses and its relevance to neurodegenerative diseases.

A unique intracellular tyrosine in neuroligin-1 regulates AMPA receptor recruitment during synapse differentiation and potentiation

To better understand the molecular mechanisms by which early neuronal connections mature into synapses, we examined the impact of neuroligin-1 (Nlg1) phosphorylation on synapse differentiation, focusing on a unique intracellular tyrosine (Y782), which differentially regulates Nlg1 binding to PSD-95 and gephyrin. By expressing Nlg1 point mutants (Y782A/F) in hippocampal neurons, we show using imaging and electrophysiology that Y782 modulates the recruitment of functional AMPA receptors (AMPARs). Nlg1-Y782F impaired both dendritic spine formation and AMPAR diffusional trapping, but not NMDA receptor recruitment, revealing the assembly of silent synapses. Furthermore, replacing endogenous Nlg1 with either Nlg1-Y782A or -Y782F in CA1 hippocampal neurons impaired long-term potentiation (LTP), demonstrating a critical role of AMPAR synaptic retention. Screening of tyrosine kinases combined with pharmacological inhibitors point to Trk family members as major regulators of endogenous Nlg1 phosphorylation and synaptogenic function. Thus, Nlg1 tyrosine phosphorylation signaling is a critical event in excitatory synapse differentiation and LTP.

Neuroligins are postsynaptic cell adhesion molecules thought to play roles in synaptic development and function. Here, authors show that phosphorylation of Y782 in neuroligin-1 modulates its role in differentiation and ability to recruit AMPARs including during long-term potentiation.

Gradients of Rac1 Nanoclusters Support Spatial Patterns of Rac1 Signaling

Rac1 is a small RhoGTPase switch that orchestrates actin branching in space and time and protrusion/retraction cycles of the lamellipodia at the cell front during mesenchymal migration. Biosensor imaging has revealed a graded concentration of active GTP-loaded Rac1 in protruding regions of the cell. Here, using single-molecule imaging and super-resolution microscopy, we show an additional supramolecular organization of Rac1. We find that Rac1 partitions and is immobilized into nanoclusters of 50-100 molecules each. These nanoclusters assemble because of the interaction of the polybasic tail of Rac1 with the phosphoinositide lipids PIP2 and PIP3. The additional interactions with GEFs and possibly GAPs, downstream effectors, and other partners are responsible for an enrichment of Rac1 nanoclusters in protruding regions of the cell. Our results show that subcellular patterns of Rac1 activity are supported by gradients of signaling nanodomains of heterogeneous molecular composition, which presumably act as discrete signaling platforms.

The Role of Actin Cytoskeleton in Dendritic Spines in the Maintenance of Long-Term Memory

Evidence indicates that long-term memory formation involves alterations in synaptic efficacy produced by modifications in neural transmission and morphology. However, it is not clear how such alterations induced by learning, that encode memory, are maintained over long period of time to preserve long-term memory. This is especially intriguing as the half-life of most of the proteins that underlie such changes is usually in the range of hours to days and these proteins may change their location over time. In this review we describe studies that indicate the involvement of dendritic spines in memory formation and its maintenance. These studies show that learning leads to changes in the number and morphology of spines. Disruption in spines morphology or manipulations that lead to alteration in their number after consolidation are associated with impairment in memory maintenance. We further ask how changes in dendritic spines morphology, induced by learning and reputed to encode memory, are maintained to preserve long-term memory. We propose a mechanism, based on studies described in the review, whereby the actin cytoskeleton and its regulatory proteins involved in the initial alteration in spine morphology induced by learning are also essential for spine structural stabilization that maintains long-term memory. In this model glutamate receptors and other synaptic receptors activation during learning leads to the creation of new actin cytoskeletal scaffold leading to changes in spines morphology and memory formation. This new actin cytoskeletal scaffold is preserved beyond actin and its regulatory proteins turnover and dynamics by active stabilization of the level and activity of actin regulatory proteins within these memory spines.

Caldendrin Directly Couples Postsynaptic Calcium Signals to Actin Remodeling in Dendritic Spines

Compartmentalization of calcium-dependent plasticity allows for rapid actin remodeling in dendritic spines. However, molecular mechanisms for the spatio-temporal regulation of filamentous actin (F-actin) dynamics by spinous Ca²⁺-transients are still poorly defined. We show that the postsynaptic Ca²⁺ sensor caldendrin orchestrates nano-domain actin dynamics that are essential for actin remodeling in the early phase of long-term potentiation (LTP). Steep elevation in spinous [Ca²⁺]_i disrupts an intramolecular interaction of caldendrin and allows cortactin binding. The fast on and slow off rate of this interaction keeps cortactin in an active conformation, and protects F-actin at the spine base against cofilin-induced severing. Caldendrin gene knockout results in higher synaptic actin turnover, altered nanoscale organization of spinous F-actin, defects in structural spine plasticity, LTP, and hippocampus-dependent learning. Collectively, the data indicate that caldendrin-cortactin directly couple [Ca²⁺]_i to preserve a minimal F-actin pool that is required for actin remodeling in the early phase of LTP.

Aβ mediates F-actin disassembly in dendritic spines leading to cognitive deficits in Alzheimer's disease

Dendritic spine loss is recognized as an early feature of Alzheimer's disease (AD), but the underlying mechanisms are poorly understood. Dendritic spine structure is defined by filamentous actin (F-actin) and we observed depolymerization of synaptosomal F-actin accompanied by increased globular-actin (G-actin) at as early as 1 month of age in a mouse model of AD (APPswe/PS1ΔE9, male mice). This led to recall deficit after contextual fear conditioning (cFC) at 2 months of age in APPswe/PS1ΔE9 male mice, which could be reversed by the actin-polymerizing agent jasplakinolide. Further, the F-actin-depolymerizing agent latrunculin induced recall deficit after cFC in WT mice, indicating the importance of maintaining F-/G-actin equilibrium for optimal behavioral response. Using direct stochastic optical reconstruction microscopy (dSTORM), we show that F-actin depolymerization in spines leads to a breakdown of the nano-organization of outwardly radiating F-actin rods in cortical neurons from APPswe/PS1ΔE9 mice. Our results demonstrate that synaptic dysfunction seen as F-actin disassembly occurs very early, before onset of pathological hallmarks in AD mice, and contributes to behavioral dysfunction, indicating that depolymerization of F-actin is causal and not consequent to decreased spine density. Further, we observed decreased synaptosomal F-actin levels in postmortem brain from mild cognitive impairment and AD patients compared with subjects with normal cognition. F-actin decrease correlated inversely with increasing AD pathology (Braak score, Aβ load, and tangle density) and directly with performance in episodic and working memory tasks, suggesting its role in human disease pathogenesis and progression.SIGNIFICANCE STATEMENT Synaptic dysfunction underlies cognitive deficits in Alzheimer's disease (AD). The cytoskeletal protein actin plays a critical role in maintaining structure and function of synapses. Using cultured neurons and an AD mouse model, we show for the first time that filamentous actin (F-actin) is lost selectively from synapses early in the disease process, long before the onset of classical AD pathology. We also demonstrate that loss of synaptic F-actin contributes directly to memory deficits. Loss of synaptosomal F-actin in human postmortem tissue correlates directly with decreased performance in memory test and inversely with AD pathology. Our data highlight that synaptic cytoarchitectural changes occur early in AD and they may be targeted for the development of therapeutics.

Using Single-Protein Tracking to Study Cell Migration

To get a complete understanding of cell migration, it is critical to study its orchestration at the molecular level. Since the recent developments in single-molecule imaging, it is now possible to study molecular phenomena at the single-molecule level inside living cells. In this chapter, we describe how such approaches have been and can be used to decipher molecular mechanisms involved in cell migration.

The Arp2/3 Regulatory System and Its Deregulation in Cancer

The Arp2/3 complex is an evolutionary conserved molecular machine that generates branched actin networks. When activated, the Arp2/3 complex contributes the actin branched junction and thus cross-links the polymerizing actin filaments in a network that exerts a pushing force. The different activators initiate branched actin networks at the cytosolic surface of different cellular membranes to promote their protrusion, movement, or scission in cell migration and membrane traffic. Here we review the structure, function, and regulation of all the direct regulators of the Arp2/3 complex that induce or inhibit the initiation of a branched actin network and that controls the stability of its branched junctions. Our goal is to present recent findings concerning novel inhibitory proteins or the regulation of the actin branched junction and place these in the context of what was previously known to provide a global overview of how the Arp2/3 complex is regulated in human cells. We focus on the human set of Arp2/3 regulators to compare normal Arp2/3 regulation in untransformed cells to the deregulation of the Arp2/3 system observed in patients affected by various cancers. In many cases, these deregulations promote cancer progression and have a direct impact on patient survival.

The Nanoworld of the Tripartite Synapse: Insights from Super-Resolution Microscopy

Synaptic connections between individual nerve cells are fundamental to the process of information transfer and storage in the brain. Over the past decades a third key partner of the synaptic machinery has been unveiled: ultrathin processes of electrically passive astroglia which often surround pre- and postsynaptic structures. The recent advent of super-resolution (SR) microscopy has begun to uncover the dynamic nanoworld of synapses and their astroglial environment. Here we overview and discuss the current progress in our understanding of the synaptic nanoenvironment, as gleaned from the imaging methods that go beyond the diffraction limit of conventional light microscopy. We argue that such methods are essential to achieve a new level of comprehension pertinent to the principles of signal integration in the brain.

Localization-based super-resolution imaging meets high-content screening

Single-molecule localization microscopy techniques have proven to be essential tools for quantitatively monitoring biological processes at unprecedented spatial resolution. However, these techniques are very low throughput and are not yet compatible with fully automated, multiparametric cellular assays. This shortcoming is primarily due to the huge amount of data generated during imaging and the lack of software for automation and dedicated data mining. We describe an automated quantitative single-molecule-based super-resolution methodology that operates in standard multiwell plates and uses analysis based on high-content screening and data-mining software. The workflow is compatible with fixed- and live-cell imaging and allows extraction of quantitative data like fluorophore photophysics, protein clustering or dynamic behavior of biomolecules. We demonstrate that the method is compatible with high-content screening using 3D dSTORM and DNA-PAINT based super-resolution microscopy as well as single-particle tracking.

Super-resolution microscopy reveals functional organization of dopamine transporters into cholesterol and neuronal activity-dependent nanodomains

Dopamine regulates reward, cognition, and locomotor functions. By mediating rapid reuptake of extracellular dopamine, the dopamine transporter is critical for spatiotemporal control of dopaminergic neurotransmission. Here, we use super-resolution imaging to show that the dopamine transporter is dynamically sequestrated into cholesterol-dependent nanodomains in the plasma membrane of presynaptic varicosities and neuronal projections of dopaminergic neurons. Stochastic optical reconstruction microscopy reveals irregular dopamine transporter nanodomains (∼70 nm mean diameter) that were highly sensitive to cholesterol depletion. Live photoactivated localization microscopy shows a similar dopamine transporter membrane organization in live heterologous cells. In neurons, dual-color dSTORM shows that tyrosine hydroxylase and vesicular monoamine transporter-2 are distinctively localized adjacent to, but not overlapping with, the dopamine transporter nanodomains. The molecular organization of the dopamine transporter in nanodomains is reversibly reduced by short-term activation of NMDA-type ionotropic glutamate receptors, implicating dopamine transporter nanodomain distribution as a potential mechanism to modulate dopaminergic neurotransmission in response to excitatory input.The dopamine transporter (DAT) has a crucial role in the regulation of neurotransmission. Here, the authors use super-resolution imaging to show that DAT clusters into cholesterol-dependent membrane regions that are reversibly regulated by ionotropic glutamate receptors activation.

Rac1 GTPase activates the WAVE regulatory complex through two distinct binding sites

The Rho GTPase Rac1 activates the WAVE regulatory complex (WRC) to drive Arp2/3 complex-mediated actin polymerization, which underpins diverse cellular processes. Here we report the structure of a WRC-Rac1 complex determined by cryo-electron microscopy. Surprisingly, Rac1 is not located at the binding site on the Sra1 subunit of the WRC previously identified by mutagenesis and biochemical data. Rather, it binds to a distinct, conserved site on the opposite end of Sra1. Biophysical and biochemical data on WRC mutants confirm that Rac1 binds to both sites, with the newly identified site having higher affinity and both sites required for WRC activation. Our data reveal that the WRC is activated by simultaneous engagement of two Rac1 molecules, suggesting a mechanism by which cells may sense the density of active Rac1 at membranes to precisely control actin assembly.

Our cells contain a network of filaments made up of a protein called actin. Just like the skeleton that supports our body, the actin ‘cytoskeleton’ gives a cell its shape and strength. Actin filaments are also critical for many other processes including enabling cells to move and divide. The assembly of actin filaments must be properly controlled so that they are formed at the right time and place within the cell.

A complex of proteins known as the WAVE Regulatory Complex (WRC) promotes the assembly of actin filaments. The complex contains a region called the VCA, which is able to bind to and activate another protein to make the new actin filaments. The WRC regulates filament assembly by controlling the availability of the VCA in a way that is similar to opening and closing a safe box. When new actin filaments are not needed, the safe box is closed and the VCA is not available. However, when cells need to make new actin filaments, the WRC is opened to release the VCA region so that it is able to bind to the filament-producing protein.

Previous studies have shown that a protein called Rac1 acts as a key to open the WRC and trigger actin filament assembly. But it remains unclear how this works. A major obstacle to studying this process is that Rac1 and the WRC only weakly interact with each other, which makes it difficult to capture the interaction under experimental conditions.

To overcome this obstacle, Chen et al. tethered a Rac1 molecule to the WRC in order to make the interaction more stable. A technique called cryo-electron microscopy was used to study the three-dimensional shape of this Rac1-WRC complex. Unexpectedly, Rac1 was attached to a different part of the WRC than the site predicted by previous studies. Further experiments showed that Rac1 needs to bind to both of these sites at the same time in order to open the WRC and release VCA, similar to using two keys to open one safe box for increased security.

Some cancers, neurological disorders and other diseases can be caused by defects in WRC and Rac1 activity. Therefore, these findings could lead to new ways to treat these conditions in human patients.

Fluorescence nanoscopy in cell biology

Fluorescence nanoscopy uniquely combines minimally invasive optical access to the internal nanoscale structure and dynamics of cells and tissues with molecular detection specificity. While the basic physical principles of 'super-resolution' imaging were discovered in the 1990s, with initial experimental demonstrations following in 2000, the broad application of super-resolution imaging to address cell-biological questions has only more recently emerged. Nanoscopy approaches have begun to facilitate discoveries in cell biology and to add new knowledge. One current direction for method improvement is the ambition to quantitatively account for each molecule under investigation and assess true molecular colocalization patterns via multi-colour analyses. In pursuing this goal, the labelling of individual molecules to enable their visualization has emerged as a central challenge. Extending nanoscale imaging into (sliced) tissue and whole-animal contexts is a further goal. In this Review we describe the successes to date and discuss current obstacles and possibilities for further development.

More than a mere supply of monomers: G-Actin pools regulate actin dynamics in dendritic spines

Katalin Schlett previews the study by Lei et al., which reveals that dendritic spines contain local pools of G-actin that are dynamically regulated in response to synaptic activity by profilin and PIP₃.

Synaptic activity reshapes the morphology of dendritic spines via regulating F-actin arborization. In this issue, Lei et al. (2017. J. Cell Biol.https://doi.org/10.1083/jcb.201612042) reports a novel, G-actin–dependent regulation of actin polymerization within spine heads. They show that actin monomer levels are elevated in spines upon activity, with G-actin immobilized by the local enrichment of phosphatidylinositol (3,4,5)-triphosphate (PIP₃) within the spine plasma membrane.

Global treadmilling coordinates actin turnover and controls the size of actin networks

Various cellular processes (including cell motility) are driven by the regulated, polarized assembly of actin filaments into distinct force-producing arrays of defined size and architecture. Branched, linear, contractile and cytosolic arrays coexist in vivo, and cells intricately control the number, length and assembly rate of filaments in these arrays. Recent in vitro and in vivo studies have revealed novel molecular mechanisms that regulate the number of filament barbed and pointed ends and their respective assembly and disassembly rates, thus defining classes of dynamically different filaments, which coexist in the same cell. We propose that a global treadmilling process, in which a steady-state amount of polymerizable actin monomers is established by the dynamics of each network, is responsible for defining the size and turnover of coexisting actin networks. Furthermore, signal-induced changes in the partitioning of actin to distinct arrays (mediated by RHO GTPases) result in the establishment of various steady-state concentrations of polymerizable monomers, thereby globally influencing the growth rate of actin filaments.

Biophysical model of the role of actin remodeling on dendritic spine morphology

Dendritic spines are small membranous structures that protrude from the neuronal dendrite. Each spine contains a synaptic contact site that may connect its parent dendrite to the axons of neighboring neurons. Dendritic spines are markedly distinct in shape and size, and certain types of stimulation prompt spines to evolve, in fairly predictable fashion, from thin nascent morphologies to the mushroom-like shapes associated with mature spines. It is well established that the remodeling of spines is strongly dependent upon the actin cytoskeleton inside the spine. A general framework that details the precise role of actin in directing the transitions between the various spine shapes is lacking. We address this issue, and present a quantitative, model-based scenario for spine plasticity validated using realistic and physiologically relevant parameters. Our model points to a crucial role for the actin cytoskeleton. In the early stages of spine formation, the interplay between the elastic properties of the spine membrane and the protrusive forces generated in the actin cytoskeleton propels the incipient spine. In the maturation stage, actin remodeling in the form of the combined dynamics of branched and bundled actin is required to form mature, mushroom-like spines. Importantly, our model shows that constricting the spine-neck aids in the stabilization of mature spines, thus pointing to a role in stabilization and maintenance for additional factors such as ring-like F-actin structures. Taken together, our model provides unique insights into the fundamental role of actin remodeling and polymerization forces during spine formation and maturation.

PTEN: Local and Global Modulation of Neuronal Function in Health and Disease

Phosphatase and tensin homolog deleted on chromosome ten (PTEN) was recently revealed to be a synaptic player during plasticity events in addition to its well-established role as a general controlling factor in cell proliferation and neuronal growth during development. Alterations of these direct actions of PTEN at synapses may lead to synaptic dysfunction with behavioral and cognitive consequences. A recent paradigmatic example of this situation, Alzheimer's disease (AD), is associated with excessive recruitment of PTEN into synapses leading to pathological synaptic depression. By contrast, some forms of autism are characterized by failure to weaken synaptic connections, which may be related to insufficient PTEN signaling. Understanding the modulation of synaptic function by PTEN in these pathologies may contribute to the development of new therapies.

Fluorescence imaging of synapse dynamics in normal circuit maturation and in developmental disorders

One of the most fundamental questions in neurobiology is how proper synaptic connections are established in the developing brain. Live-cell imaging of the synaptic structure and functional molecules can reveal the time course of synapse formation, molecular dynamics, and functional maturation. Using postsynaptic scaffolding proteins as a marker of synapse development, fluorescence time-lapse imaging revealed rapid formation of individual synapses that occurred within hours and their remodeling in culture preparations. In vivo two-photon excitation microscopy development enabled us to directly measure synapse turnover in living animals. In vivo synapse dynamics were suppressed in the adult rodent brain, but were maintained at a high level during the early postnatal period. This transition in synapse dynamics is biologically important and can be linked to the pathology of juvenile-onset psychiatric diseases. Indeed, the upregulation of synapse dynamics was observed in multiple mouse models of autism spectrum disorders. Fluorescence imaging of synapses provides new information regarding the physiology and pathology of neural circuit construction.

Defective actin dynamics in dendritic spines: cause or consequence of age-induced cognitive decline?

Ageing is a complex deteriorating process that coincides with changes in metabolism, replicative senescence, increased resistance to apoptosis, as well as progressive mitochondria dysfunction that lead to an increase production and accumulation of reactive oxygen species (ROS). Although controversy on the paradigm of the oxidative damage theory of ageing exists, persuasive studies in Caenorhabditis elegans and yeast have demonstrated that manipulation of ROS can modify the process of ageing and influences the damage of proteins, lipids and DNA. In neurons, ageing impacts on the intrinsic neuronal excitability, it decreases the size of neuronal soma and induces the loss of dendrites and dendritic spines. The actin cytoskeleton is an abundant and broadly expressed system that plays critical functions in many cellular processes ranging from cell motility to controlling cell shape and polarity. It is thus hardly surprising that the expression and the function of actin in neurons is crucial for the morphological changes that occur in the brain throughout life. We propose that alterations in actin filament dynamics in dendritic spines may be one of the key events contributing to the initial phases of ageing in the brain.

Nanoscale organization of synaptic adhesion proteins revealed by single-molecule localization microscopy

The advent of superresolution imaging has created a strong need for both optimized labeling strategies and analysis methods to probe the nanoscale organization of complex biological structures. We present a thorough description of the distribution of synaptic adhesion proteins at the nanoscopic scale, namely presynaptic neurexin-[Formula: see text] ([Formula: see text]), and its two postsynaptic binding partners neuroligin-1 (Nlg1) and leucine-rich-repeat transmembrane protein 2 (LRRTM2). We monitored these proteins in the membrane of neurons by direct stochastic optical reconstruction microscopy, after live surface labeling with Alexa647-conjugated monomeric streptavidin. The small probe ([Formula: see text]) efficiently penetrates into crowded synaptic junctions and reduces the distance to target. We quantified the organization of the single-molecule localization data using a tesselation-based analysis technique. We show that Nlg1 exhibits a fairly disperse organization within dendritic spines, while LRRTM2 is organized in compact domains, and [Formula: see text] in presynaptic terminals displays a dual-organization pattern intermediate between that of Nlg1 and LRRTM2. These results suggest that part of [Formula: see text] interacts transsynaptically with Nlg1 and the other part with LRRTM2.

Protein Crowding within the Postsynaptic Density Can Impede the Escape of Membrane Proteins

Mechanisms regulating lateral diffusion and positioning of glutamate receptors within the postsynaptic density (PSD) determine excitatory synaptic strength. Scaffold proteins in the PSD are abundant receptor binding partners, yet electron microscopy suggests that the PSD is highly crowded, potentially restricting the diffusion of receptors regardless of binding. However, the contribution of macromolecular crowding to receptor retention remains poorly understood. We combined experimental and computational approaches to test the effect of synaptic crowding on receptor movement and positioning in Sprague Dawley rat hippocampal neurons. We modeled AMPA receptor diffusion in synapses where the distribution of scaffold proteins was determined from photoactivated localization microscopy experiments, and receptor-scaffold association and dissociation rates were adjusted to fit single-molecule tracking and fluorescence recovery measurements. Simulations predicted that variation of receptor size strongly influences the fractional synaptic area the receptor may traverse, and the proportion that may exchange in and out of the synapse. To test the model experimentally, we designed a set of novel transmembrane (TM) probes. A single-pass TM protein with one PDZ binding motif concentrated in the synapse as do AMPARs yet was more mobile there than the much larger AMPAR. Furthermore, either the single binding motif or an increase in cytoplasmic bulk through addition of a single GFP slowed synaptic movement of a small TM protein. These results suggest that both crowding and binding limit escape of AMPARs from the synapse. Moreover, tight protein packing within the PSD may modulate the synaptic dwell time of many TM proteins important for synaptic function.

SIGNIFICANCE STATEMENT

Small alterations to the distribution within synapses of key transmembrane proteins, such as receptors, can dramatically change synaptic strength. Indeed, many diseases are thought to unbalance neural circuit function in this manner. Processes that regulate this in healthy synapses are unclear, however. By combining computer simulations with imaging methods that examined protein dynamics at multiple scales in space and time, we showed that both steric effects and protein-protein binding each regulate the mobility of receptors in the synapse. Our findings extend our knowledge of the synapse as a crowded environment that counteracts molecular diffusion, and support the idea that both molecular collisions and biochemical binding can be involved in the regulation of neural circuit performance.

Organization and dynamics of the actin cytoskeleton during dendritic spine morphological remodeling

In the central nervous system, most excitatory post-synapses are small subcellular structures called dendritic spines. Their structure and morphological remodeling are tightly coupled to changes in synaptic transmission. The F-actin cytoskeleton is the main driving force of dendritic spine remodeling and sustains synaptic plasticity. It is therefore essential to understand how changes in synaptic transmission can regulate the organization and dynamics of actin binding proteins (ABPs). In this review, we will provide a detailed description of the organization and dynamics of F-actin and ABPs in dendritic spines and will discuss the current models explaining how the actin cytoskeleton sustains both structural and functional synaptic plasticity.

Super resolution microscopy is poised to reveal new insights into the formation and maturation of dendritic spines

Dendritic spines and synapses are critical for neuronal communication, and they are perturbed in many neurological disorders; however, the study of these structures in living cells has been hindered by their small size. Super resolution microscopy, unlike conventional light microscopy, is diffraction unlimited and thus is well suited for imaging small structures, such as dendritic spines and synapses. Super resolution microscopy has already revealed important new information about spine and synapse morphology, actin remodeling, and nanodomain composition in both healthy cells and diseased states. In this review, we highlight the advancements in probes that make super resolution more amenable to live-cell imaging of spines and synapses. We also discuss recent data obtained by super resolution microscopy that has advanced our knowledge of dendritic spine and synapse structure, organization, and dynamics in both healthy and diseased contexts. Finally, we propose a series of critical questions for understanding spine and synapse formation and maturation that super resolution microscopy is poised to answer.

Cyfip1 Regulates Presynaptic Activity during Development

Copy number variations encompassing the gene encoding Cyfip1 have been associated with a variety of human diseases, including autism and schizophrenia. Here we show that juvenile mice hemizygous for Cyfip1 have altered presynaptic function, enhanced protein translation, and increased levels of F-actin. In developing hippocampus, reduced Cyfip1 levels serve to decrease paired pulse facilitation and increase miniature EPSC frequency without a change in amplitude. Higher-resolution examination shows these changes to be caused primarily by an increase in presynaptic terminal size and enhanced vesicle release probability. Short hairpin-mediated knockdown of Cyfip1 coupled with expression of mutant Cyfip1 proteins indicates that the presynaptic alterations are caused by dysregulation of the WAVE regulatory complex. Such dysregulation occurs downstream of Rac1 as acute exposure to Rac1 inhibitors rescues presynaptic responses in culture and in hippocampal slices. The data serve to highlight an early and essential role for Cyfip1 in the generation of normally functioning synapses and suggest a means by which changes in Cyfip1 levels could impact the generation of neural networks and contribute to abnormal and maladaptive behaviors.

SIGNIFICANCE STATEMENT

Several developmental brain disorders have been associated with gene duplications and deletions that serve to increase or decrease levels of encoded proteins. Cyfip1 is one such protein, but the role it plays in brain development is poorly understood. We asked whether decreased Cyfip1 levels altered the function of developing synapses. The data show that synapses with reduced Cyfip1 are larger and release neurotransmitter more rapidly. These effects are due to Cyfip1's role in actin polymerization and are reversed by expression of a Cyfip1 mutant protein retaining actin regulatory function or by inhibiting Rac1. Thus, Cyfip1 has a more prominent early role regulating presynaptic activity during a stage of development when activity helps to define neural pathways.

Focusing super resolution on the cytoskeleton

Super resolution imaging is becoming an increasingly important tool in the arsenal of methods available to cell biologists. In recognition of its potential, the Nobel Prize for chemistry was awarded to three investigators involved in the development of super resolution imaging methods in 2014. The availability of commercial instruments for super resolution imaging has further spurred the development of new methods and reagents designed to take advantage of super resolution techniques. Super resolution offers the advantages traditionally associated with light microscopy, including the use of gentle fixation and specimen preparation methods, the ability to visualize multiple elements within a single specimen, and the potential to visualize dynamic changes in living specimens over time. However, imaging of living cells over time is difficult and super resolution imaging is computationally demanding. In this review, we discuss the advantages/disadvantages of different super resolution systems for imaging fixed live specimens, with particular regard to cytoskeleton structures.

Coordinating neuronal actin-microtubule dynamics

The growth and migration of neurons require continuous remodelling of the neuronal cytoskeleton, providing a versatile cellular framework for force generation and guided movement, in addition to structural support. Actin filaments and microtubules are central to the dynamic action of the cytoskeleton and rapid advances in imaging technologies are enabling ever more detailed visualisation of the dynamic intracellular networks that they form. However, these filaments do not act individually and an expanding body of evidence emphasises the importance of actin-microtubule crosstalk in orchestrating cytoskeletal dynamics. Here, we summarise our current understanding of the structure and dynamics of actin and microtubules in isolation, before reviewing both the mechanisms and the molecular players involved in mediating actin-microtubule crosstalk in neurons.

Actin cytoskeleton in dendritic spine development and plasticity

Synapses are the basic unit of neuronal communication and their disruption is associated with many neurological disorders. Significant progress has been made towards understanding the molecular and genetic regulation of synapse formation, modulation, and dysfunction, but the underlying cellular mechanisms remain incomplete. The actin cytoskeleton not only provides the structural foundation for synapses, but also regulates a diverse array of cellular activities underlying synaptic function. Here we will discuss the regulation of the actin cytoskeleton in dendritic spines, the postsynaptic compartment of excitatory synapses. We will focus on a select number of actin regulatory processes, highlighting recent advances, the complexity of crosstalk between different pathways, and the challenges of understanding their precise impact on the structure and function of synapses.

Regulation of the Postsynaptic Compartment of Excitatory Synapses by the Actin Cytoskeleton in Health and Its Disruption in Disease

Disruption of synaptic function at excitatory synapses is one of the earliest pathological changes seen in wide range of neurological diseases. The proper control of the segregation of neurotransmitter receptors at these synapses is directly correlated with the intact regulation of the postsynaptic cytoskeleton. In this review, we are discussing key factors that regulate the structure and dynamics of the actin cytoskeleton, the major cytoskeletal building block that supports the postsynaptic compartment. Special attention is given to the complex interplay of actin-associated proteins that are found in the synaptic specialization. We then discuss our current understanding of how disruption of these cytoskeletal elements may contribute to the pathological events observed in the nervous system under disease conditions with a particular focus on Alzheimer's disease pathology.

Shank-cortactin interactions control actin dynamics to maintain flexibility of neuronal spines and synapses

The family of Shank scaffolding molecules (comprising Shank1, 2 and 3) are core components of the postsynaptic density (PSD) in neuronal synapses. Shanks link surface receptors to other scaffolding molecules within the PSD, as well as to the actin cytoskeleton. However, determining the function of Shank proteins in neurons has been complicated because the different Shank isoforms share a very high degree of sequence and domain homology. Therefore, to control Shank content while minimizing potential compensatory effects, a miRNA-based knockdown strategy was developed to reduce the expression of all synaptically targeted Shank isoforms simultaneously in rat hippocampal neurons. Using this approach, a strong (>75%) reduction in total Shank protein levels was achieved at individual dendritic spines, prompting an approximately 40% decrease in mushroom spine density. Furthermore, Shank knockdown reduced spine actin levels and increased sensitivity to the actin depolymerizing agent Latrunculin A. A SHANK2 mutant lacking the proline-rich cortactin-binding motif (SHANK2-ΔPRO) was unable to rescue these defects. Furthermore, Shank knockdown reduced cortactin levels in spines and increased the mobility of spine cortactin as measured by single-molecule tracking photoactivated localization microscopy, suggesting that Shank proteins recruit and stabilize cortactin at the synapse. Furthermore, it was found that Shank knockdown significantly reduced spontaneous remodelling of synapse morphology that could not be rescued by the SHANK2-ΔPRO mutant. It was concluded that Shank proteins are key intermediates between the synapse and the spine interior that, via cortactin, permit the actin cytoskeleton to dynamically regulate synapse morphology and function.

Meeting report--Imaging the Cell

Every two years, the French Society for Cell Biology (SBCF) organises an international meeting called 'Imaging the Cell'. This year, the 8th edition was held on 24-26 June 2015 at University of Bordeaux Campus Victoire in the city of Bordeaux, France, a UNESCO World Heritage site. Over the course of three days, the meeting provided a forum for experts in different areas of cell imaging. Its unique approach was to combine conventional oral presentations during morning sessions with practical workshops at hosting institutes and the Bordeaux Imaging Center during the afternoons. The meeting, co-organised by Violaine Moreau and Frédéric Saltel (both INSERM U1053, Bordeaux, France), Christel Poujol and Fabrice Cordelières (both Bordeaux Imaging Center, Bordeaux, France) and Isabelle Sagot (Institut de Biochimie et Génétique Cellulaires, Bordeaux, France), brought together about 120 scientists including 16 outstanding speakers to discuss the latest advances in cell imaging. Thanks to recent progress in imaging technologies, cell biologists are now able to visualise, follow and manipulate cellular processes with unprecedented accuracy. The meeting sessions and workshops highlighted some of the most exciting developments in the field, with sessions dedicated to optogenetics, high-content screening, in vivo and live-cell imaging, correlative light and electron microscopy, as well as super-resolution imaging.

Actin Out: Regulation of the Synaptic Cytoskeleton

The small size of dendritic spines belies the elaborate role they play in excitatory synaptic transmission and ultimately complex behaviors. The cytoskeletal architecture of the spine is predominately composed of actin filaments. These filaments, which at first glance might appear simple, are also surprisingly complex. They dynamically assemble into different structures and serve as a platform for orchestrating the elaborate responses of the spine during spinogenesis and experience-dependent plasticity. Multiple mutations associated with human neurodevelopmental and psychiatric disorders involve genes that encode regulators of the synaptic cytoskeleton. A major, unresolved question is how the disruption of specific actin filament structures leads to the onset and progression of complex synaptic and behavioral phenotypes. This review will cover established and emerging mechanisms of actin cytoskeletal remodeling and how this influences specific aspects of spine biology that are implicated in disease.

Competition and collaboration between different actin assembly pathways allows for homeostatic control of the actin cytoskeleton

Tremendous insight into actin-associated proteins has come from careful biochemical and cell biological characterization of their activities and regulation. However, many studies of their cellular behavior have only considered each in isolation. Recent efforts reveal that assembly factors compete for polymerization-competent actin monomers, suggesting that actin is homeostatically regulated. It seems that a major regulatory component is competition between Arp2/3-activating nucleation promoting factors and profilin for actin monomers. The result is differential delivery of actin to different pathways, allowing for simultaneous assembly of competing F-actin structures and collaborative building of higher order cellular structures. Although there are likely to be additional factors that regulate actin homeostasis, especially in a cell type-dependent fashion, we advance the notion that competition between actin assembly factors results in a tunable system that can be adjusted according to extracellular and intracellular cues.

Biochemical Computation for Spine Structural Plasticity

The structural plasticity of dendritic spines is considered to be essential for various forms of synaptic plasticity, learning, and memory. The process is mediated by a complex signaling network consisting of numerous species of molecules. Furthermore, the spatiotemporal dynamics of the biochemical signaling are regulated in a complicated manner because of geometrical restrictions from the unique morphology of the dendritic branches and spines. Recent advances in optical techniques have enabled the exploration of the spatiotemporal aspects of the signal regulations in spines and dendrites and have provided many insights into the principle of the biochemical computation that underlies spine structural plasticity.

Studying neuronal microtubule organization and microtubule-associated proteins using single molecule localization microscopy

The formation and maintenance of highly polarized neurons critically depends on the proper organization of the microtubule (MT) cytoskeleton. In axons, MTs are uniformly oriented with their plus-end pointing outward whereas in mature dendrites MTs have mixed orientations. MT organization and dynamics can be regulated by MT-associated proteins (MAPs). Plus-end tracking proteins are specialized MAPs that decorate plus-ends of growing MTs and regulate neuronal polarity, neurite extension, and dendritic spine morphology. Conventional fluorescence microscopy enables observation of specific cellular components through molecule-specific labeling but provides limited resolution (∼250 nm). Therefore, electron microscopy has until now provided most of our knowledge about the precise MT organization in neurons. In the past decade, super-resolution fluorescence microscopy techniques have emerged that circumvent the diffraction limit of light and enable high-resolution reconstruction of the MT network combined with selective protein labeling. However, preserving MT ultrastructure, MAP binding, high labeling density, and antibody specificity after fixation protocols is still quite challenging. In this chapter, we provide an optimized protocol for two-color direct stochastic optical reconstruction microscopy imaging of neuronal MTs together with their growing plus-ends to probe MT architecture and polarity.