Curated Optogenetic Publication Database

Abstract: The structural plasticity of synapses is crucial for regulating brain functions. However, currently available methods for studying synapse organization based on split fluorescent proteins (FPs) have been limited in assessing synaptic dynamics in vivo due to the irreversible binding of split FPs. Here, we develop 'SynapShot', a method for visualizing the structural dynamics of intact synapses by combining dimerization-dependent FPs (ddFPs) with engineered synaptic adhesion molecules. SynapShot allows real-time monitoring of reversible and bidirectional changes of synaptic contacts under physiological stimulation. The application of green and red ddFPs in SynapShot enables simultaneous visualization of two distinct populations of synapses. Notably, the red-shifted SynapShot is highly compatible with blue light-based optogenetic techniques, allowing for visualization of synaptic dynamics while precisely controlling specific signaling pathways. Furthermore, we demonstrate that SynapShot enables real-time monitoring of structural changes in synaptic contacts in the mouse brain during both primitive and higher-order behaviors.

Abstract: Proteases function as pivotal molecular switches, initiating numerous biological events. Notably, potyviral protease, derived from plant viruses, has emerged as a trusted proteolytic switch in synthetic biological circuits. To harness their capabilities, we have developed a single-component photocleavable switch, termed LAUNCHER (Light-Assisted UNcaging switCH for Endoproteolytic Release), by employing a circularly permutated tobacco etch virus protease and a blue-light-gated substrate, which are connected by fine-tuned intermodular linkers. As a single-component system, LAUNCHER exhibits a superior signal-to-noise ratio compared with multi-component systems, enabling precise and user-controllable release of payloads. This characteristic renders LAUNCHER highly suitable for diverse cellular applications, including transgene expression, tailored subcellular translocation and optochemogenetics. Additionally, the plug-and-play integration of LAUNCHER into existing synthetic circuits facilitates the enhancement of circuit performance. The demonstrated efficacy of LAUNCHER in improving existing circuitry underscores its significant potential for expanding its utilization in various applications.

Abstract: Cyclic adenosine monophosphate (cAMP) is a second messenger that mediates diverse intracellular signals. Studies of cAMP transport in cells have produced wildly different results, from reports of nearly free diffusion to reports that cAMP remains localized in nanometer-scale domains. We developed an all-optical toolkit, termed cAMP-SITES, to locally perturb and map cAMP transport. In MDCK cells and in cultured neurons, cAMP had a diffusion coefficient of ~120 μm2/s, similar to the diffusion coefficients of other small molecules in cytoplasm. In neuronal dendrites, a balance between diffusion and degradation led to cAMP domains with a length scale of ~30 μm. Geometrical confinement by membranes led to subcellular variations in cAMP concentration, but we found no evidence of nanoscale domains or of distinct membrane-local and cytoplasmic pools. We introduce theoretical relations between cell geometry and small-molecule reaction-diffusion dynamics and transport to explain our observations.

Abstract: Microtubules are cytoskeletal polymers that separate chromosomes during mitosis and serve as rails for intracellular transport and organelle positioning. Manipulation of microtubules is widely used in cell and developmental biology, but tools for precise subcellular spatiotemporal control of microtubules are currently lacking. Here, we describe a light-activated system for localized recruitment of the microtubule-severing enzyme katanin. This system, named opto-katanin, uses targeted illumination with blue light to induce rapid, localized, and reversible microtubule depolymerization. This tool allows precise clearing of a subcellular region of microtubules while preserving the rest of the microtubule network, demonstrating that regulation of katanin recruitment to microtubules is sufficient to control its severing activity. The tool is not toxic in the absence of blue light and can be used to disassemble both dynamic and stable microtubules in primary neurons as well as in dividing cells. We show that opto-katanin can be used to locally block vesicle transport and to clarify the dependence of organelle morphology and dynamics on microtubules. Specifically, our data indicate that microtubules are not required for the maintenance of the Golgi stacks or the tubules of the endoplasmic reticulum but are needed for the formation of new membrane tubules. Finally, we demonstrate that this tool can be applied to study the contribution of microtubules to cell mechanics by showing that microtubule bundles can exert forces constricting the nucleus.

Abstract: Optogenetic manipulations have transformed neuroscience in recent years. While sophisticated tools now exist for controlling the firing patterns of neurons, it remains challenging to optogenetically define the plasticity state of individual synapses. A variety of synapses in the mammalian brain express presynaptic long-term potentiation (LTP) upon elevation of presynaptic cyclic adenosine monophosphate (cAMP), but the molecular expression mechanisms as well as the impact of presynaptic LTP on network activity and behavior are not fully understood. In order to establish optogenetic control of presynaptic cAMP levels and thereby presynaptic potentiation, we developed synaptoPAC, a presynaptically targeted version of the photoactivated adenylyl cyclase bPAC. In cultures of hippocampal granule cells of Wistar rats, activation of synaptoPAC with blue light increased action potential-evoked transmission, an effect not seen in hippocampal cultures of non-granule cells. In acute brain slices of C57BL/6N mice, synaptoPAC activation immediately triggered a strong presynaptic potentiation at mossy fiber synapses in CA3, but not at Schaffer collateral synapses in CA1. Following light-triggered potentiation, mossy fiber transmission decreased within 20 min, but remained enhanced still after 30 min. The optogenetic potentiation altered the short-term plasticity dynamics of release, reminiscent of presynaptic LTP. Our work establishes synaptoPAC as an optogenetic tool that enables acute light-controlled potentiation of transmitter release at specific synapses in the brain, facilitating studies of the role of presynaptic potentiation in network function and animal behavior in an unprecedented manner. Read the Editorial Highlight for this article on page 270.

Abstract: Activation of Fas (CD95) is observed in various neurological disorders and can lead to both apoptosis and prosurvival outputs, yet how Fas signaling operates dynamically in the hippocampus is poorly understood. The optogenetic dissection of a signaling network can yield molecular-level explanations for cellular responses or fates, including the signaling dysfunctions seen in numerous diseases. Here, we developed an optogenetically activatable Fas that works in a physiologically plausible manner. Fas activation in immature neurons of the dentate gyrus triggered mammalian target of rapamycin (mTOR) activation and subsequent brain-derived neurotrophic factor secretion. Phosphorylation of extracellular signal-regulated kinase (Erk) in neural stem cells was induced under prolonged Fas activation. Repetitive activation of this signaling network yielded proliferation of neural stem cells and a transient increase in spatial working memory in mice. Our results demonstrate a novel Fas signaling network in the dentate gyrus and illuminate its consequences for adult neurogenesis and memory enhancement.

Abstract: Despite efforts to visualize the spatio-temporal dynamics of single messenger RNAs, the ability to precisely control their function has lagged. This study presents an optogenetic approach for manipulating the localization and translation of specific mRNAs by trapping them in clusters. This clustering greatly amplified reporter signals, enabling endogenous RNA-protein interactions to be clearly visualized in single cells. Functionally, this sequestration reduced the ability of mRNAs to access ribosomes, markedly attenuating protein synthesis. A spatio-temporally resolved analysis indicated that sequestration of endogenous β-actin mRNA attenuated cell motility through the regulation of focal-adhesion dynamics. These results suggest a mechanism highlighting the indispensable role of newly synthesized β-actin protein for efficient cell migration. This platform may be broadly applicable for use in investigating the spatio-temporal activities of specific mRNAs in various biological processes.

Abstract: In this study, we use an optogenetic inhibitor of JNK in dendritic spine sub-compartments of rat hippocampal neurons. JNK inhibition exerts rapid (within seconds) reorganisation of actin in the spine-head. Using real-time FRET to measure JNK activity, we find that either excitotoxic insult (NMDA) or endocrine stress (corticosterone), activate spine-head JNK causing internalization of AMPARs and spine retraction. Both events are prevented upon optogenetic inhibition of JNK, and rescued by JNK inhibition even 2 h after insult. Moreover, we identify that the fast-acting anti-depressant ketamine reduces JNK activity in hippocampal neurons suggesting that JNK inhibition may be a downstream mediator of its anti-depressant effect. In conclusion, we show that JNK activation plays a role in triggering spine elimination by NMDA or corticosterone stress, whereas inhibition of JNK facilitates regrowth of spines even in the continued presence of glucocorticoid. This identifies that JNK acts locally in the spine-head to promote AMPAR internalization and spine shrinkage following stress, and reveals a protective function for JNK inhibition in preventing spine regression.SIGNIFICANCE STATEMENT Identifying mechanisms that underlie dendritic spine elimination is important if we are to understand maladaptive changes that contribute to psychiatric disease. Compartment-specific, fast-acting tools can expedite this endeavor. Here we use a light-activated inhibitor of JNK to control kinase activity specifically in dendritic spines. Light-activation of the JNK inhibitor reduces AMPA receptor removal and spine regression in response to corticosterone and NMDA stress. Furthermore, we find that the anti-depressant drug ketamine lowers JNK activity in hippocampal neurons and prevents spine regression, though direct JNK inhibition is more effective. This study identifies a role for JNK in spine regression and may be relevant for endocrine control of synaptic strength and for conditions where chronic glucocorticoid stress leads to spine elimination.

Abstract: Actin waves are filamentous actin (F-actin)-rich structures that initiate in the somato-neuritic area and move toward neurite ends. The upstream cues that initiate actin waves are poorly understood. Here, using an optogenetic approach (Opto-cytTrkB), we found that local activation of the TrkB receptor around the neurite end initiates actin waves and triggers neurite elongation. During actin wave generation, locally activated TrkB signaling in the distal neurite was functionally connected with preferentially localized Rac1 and its signaling pathways in the proximal region. Moreover, TrkB activity changed the location of ankyrinG--the master organizer of the axonal initial segment-and initiated the stimulated neurite to acquire axonal characteristics. Taken together, these findings suggest that local Opto-cytTrkB activation switches the fate from minor to major axonal neurite during neuronal polarization by generating actin waves.

Abstract: Regulated secretion is critical for diverse biological processes ranging from immune and endocrine signaling to synaptic transmission. Botulinum and tetanus neurotoxins, which specifically proteolyze vesicle fusion proteins involved in regulated secretion, have been widely used as experimental tools to block these processes. Genetic expression of these toxins in the nervous system has been a powerful approach for disrupting neurotransmitter release within defined circuitry, but their current utility in the brain and elsewhere remains limited by lack of spatial and temporal control. Here we engineered botulinum neurotoxin B so that it can be activated with blue light. We demonstrate the utility of this approach for inducibly disrupting excitatory neurotransmission, providing a first-in-class optogenetic tool for persistent, light-triggered synaptic inhibition. In addition to blocking neurotransmitter release, this approach will have broad utility for conditionally disrupting regulated secretion of diverse bioactive molecules, including neuropeptides, neuromodulators, hormones, and immune molecules. VIDEO ABSTRACT.

Abstract: Ras and Rho small GTPases are critical for numerous cellular processes including cell division, migration, and intercellular communication. Despite extensive efforts to visualize the spatiotemporal activity of these proteins, achieving the sensitivity and dynamic range necessary for in vivo application has been challenging. Here, we present highly sensitive intensiometric small GTPase biosensors visualizing the activity of multiple small GTPases in single cells in vivo. Red-shifted sensors combined with blue light-controllable optogenetic modules achieved simultaneous monitoring and manipulation of protein activities in a highly spatiotemporal manner. Our biosensors revealed spatial dynamics of Cdc42 and Ras activities upon structural plasticity of single dendritic spines, as well as a broad range of subcellular Ras activities in the brains of freely behaving mice. Thus, these intensiometric small GTPase sensors enable the spatiotemporal dissection of complex protein signaling networks in live animals.

Abstract: Currently available inhibitory optogenetic tools provide short and transient silencing of neurons, but they cannot provide long-lasting inhibition because of the requirement for high light intensities. Here we present an optimized blue-light-sensitive synthetic potassium channel, BLINK2, which showed good expression in neurons in three species. The channel is activated by illumination with low doses of blue light, and in our experiments it remained active over (tens of) minutes in the dark after the illumination was stopped. This activation caused long periods of inhibition of neuronal firing in ex vivo recordings of mouse neurons and impaired motor neuron response in zebrafish in vivo. As a proof-of-concept application, we demonstrated that in a freely moving rat model of neuropathic pain, the activation of a small number of BLINK2 channels caused a long-lasting (>30 min) reduction in pain sensation.

Abstract: Optogenetic manipulation of cells or living organisms became widely used in neuroscience following the introduction of the light-gated ion channel channelrhodopsin-2 (ChR2). ChR2 is a non-selective cation channel, ideally suited to depolarize and evoke action potentials in neurons. However, its calcium (Ca2+) permeability and single channel conductance are low and for some applications longer-lasting increases in intracellular Ca2+ might be desirable. Moreover, there is need for an efficient light-gated potassium (K+) channel that can rapidly inhibit spiking in targeted neurons. Considering the importance of Ca2+ and K+ in cell physiology, light-activated Ca2+-permeant and K+-specific channels would be welcome additions to the optogenetic toolbox. Here we describe the engineering of novel light-gated Ca2+-permeant and K+-specific channels by fusing a bacterial photoactivated adenylyl cyclase to cyclic nucleotide-gated channels with high permeability for Ca2+ or for K+, respectively. Optimized fusion constructs showed strong light-gated conductance in Xenopus laevis oocytes and in rat hippocampal neurons. These constructs could also be used to control the motility of Drosophila melanogaster larvae, when expressed in motoneurons. Illumination led to body contraction when motoneurons expressed the light-sensitive Ca2+-permeant channel, and to body extension when expressing the light-sensitive K+ channel, both effectively and reversibly paralyzing the larvae. Further optimization of these constructs will be required for application in adult flies since both constructs led to eclosion failure when expressed in motoneurons.

Abstract: Erythropoietin-producing hepatocellular (Eph) receptors regulate a wide array of developmental processes by responding to cell-cell contacts. EphB2 is well-expressed in brain and known to be important for dendritic spine development, as well as for the maintenance of the synapses, although the mechanisms of these functions have not been fully understood. Here we studied EphB2's functions in hippocampal neurons with an optogenetic approach, which allows us to specify spatial regions of signal activation and monitor in real-time the consequences of signal activation. We designed and constructed OptoEphB2, a genetically encoded photoactivatable EphB2. Photoactivation of OptoEphB2 in fibroblast cells induced receptor phosphorylation and resulted in cell rounding - a well-known cellular response to EphB2 activation. In contrast, local activation of OptoEphb2 in dendrites of hippocampal neurons induces rapid actin polymerization, resulting dynamic dendritic filopodial growth. Inhibition of Rac1 and CDC42 did not abolish OptoEphB2-induced actin polymerization. Instead, we identified Abelson Tyrosine-Protein Kinase 2 (Abl2/Arg) as a necessary effector in OptoEphB2-induced filopodia growth in dendrites. These findings provided new mechanistic insight into EphB2's role in neural development and demonstrated the advantage of OptoEphB as a new tool for studying EphB signaling.

Abstract: Despite recent advances in optogenetics, it remains challenging to manipulate gene expression in specific populations of neurons. We present a dual-protein switch system, Cal-Light, that translates neuronal-activity-mediated calcium signaling into gene expression in a light-dependent manner. In cultured neurons and brain slices, we show that Cal-Light drives expression of the reporter EGFP with high spatiotemporal resolution only in the presence of both blue light and calcium. Delivery of the Cal-Light components to the motor cortex of mice by viral vectors labels a subset of excitatory and inhibitory neurons related to learned lever-pressing behavior. By using Cal-Light to drive expression of the inhibitory receptor halorhodopsin (eNpHR), which responds to yellow light, we temporarily inhibit the lever-pressing behavior, confirming that the labeled neurons mediate the behavior. Thus, Cal-Light enables dissection of neural circuits underlying complex mammalian behaviors with high spatiotemporal precision.

Abstract: Few tools exist to visualize and manipulate neurons that are targets of neuromodulators. We present iTango, a light- and ligand-gated gene expression system based on a light-inducible split tobacco etch virus protease. Cells expressing the iTango system exhibit increased expression of a marker gene in the presence of dopamine and blue-light exposure, both in vitro and in vivo. We demonstrated the iTango system in a behaviorally relevant context, by inducing expression of optogenetic tools in neurons under dopaminergic control during a behavior of interest. We thereby gained optogenetic control of these behaviorally relevant neurons. We applied the iTango system to decipher the roles of two classes of dopaminergic neurons in the mouse nucleus accumbens in a sensitized locomotor response to cocaine. Thus, the iTango platform allows for control of neuromodulatory circuits in a genetically and functionally defined manner with spatial and temporal precision.

Abstract: Elucidating temporal windows of signaling activity required for synaptic and behavioral plasticity is crucial for understanding molecular mechanisms underlying these phenomena. Here, we developed photoactivatable autocamtide inhibitory peptide 2 (paAIP2), a genetically encoded, light-inducible inhibitor of CaMKII activity. The photoactivation of paAIP2 in neurons for 1-2 min during the induction of LTP and structural LTP (sLTP) of dendritic spines inhibited these forms of plasticity in hippocampal slices of rodents. However, photoactivation ∼1 min after the induction did not affect them, suggesting that the initial 1 min of CaMKII activation is sufficient for inducing LTP and sLTP. Furthermore, the photoactivation of paAIP2 expressed in amygdalar neurons of mice during an inhibitory avoidance task revealed that CaMKII activity during, but not after, training is required for the memory formation. Thus, we demonstrated that paAIP2 is useful to elucidate the temporal window of CaMKII activation required for synaptic plasticity and learning.

Abstract: The molecular composition of the postsynaptic membrane is sculpted by synaptic activity. During synaptic plasticity at excitatory synapses, numerous structural, signaling, and receptor molecules concentrate at the postsynaptic density (PSD) to regulate synaptic strength. We developed an approach that uses light to tune the abundance of specific molecules in the PSD. We used this approach to investigate the relationship between the number of AMPA-type glutamate receptors in the PSD and synaptic strength. Surprisingly, adding more AMPA receptors to excitatory contacts had little effect on synaptic strength. Instead, we observed increased excitatory input through the apparent addition of new functional sites. Our data support a model where adding AMPA receptors is sufficient to activate synapses that had few receptors to begin with, but that additional remodeling events are required to strengthen established synapses. More broadly, this approach introduces the precise spatiotemporal control of optogenetics to the molecular control of synaptic function.

Abstract: Inducible dimers are powerful tools for controlling biological processes through colocalizing signaling molecules. To be effective, an inducible system should have a dissociation constant in the "off" state that is greater (i.e., weaker affinity) than the concentrations of the molecules that are being controlled, and in the "on" state a dissociation constant that is less (i.e., stronger affinity) than the relevant protein concentrations. Here, we reengineer the interaction between the light inducible dimer, iLID, and its binding partner SspB, to better control proteins present at high effective concentrations (5-100 μM). iLID contains a light-oxygen-voltage (LOV) domain that undergoes a conformational change upon activation with blue light and exposes a peptide motif, ssrA, that binds to SspB. The new variant of the dimer system contains a single SspB point mutation (A58V), and displays a 42-fold change in binding affinity when activated with blue light (from 3 ± 2 μM to 125 ± 40 μM) and allows for light-activated colocalization of transmembrane proteins in neurons, where a higher affinity switch (0.8-47 μM) was less effective because more colocalization was seen in the dark. Additionally, with a point mutation in the LOV domain (N414L), we lengthened the reversion half-life of iLID. This expanded suite of light induced dimers increases the variety of cellular pathways that can be targeted with light.

Abstract: Cyclic-AMP is one of the most important second messengers, regulating many crucial cellular events in both prokaryotes and eukaryotes, and precise spatial and temporal control of cAMP levels by light shows great promise as a simple means of manipulating and studying numerous cell pathways and processes. The photoactivated adenylate cyclase (PAC) from the photosynthetic cyanobacterium Oscillatoria acuminata (OaPAC) is a small homodimer eminently suitable for this task, requiring only a simple flavin chromophore within a blue light using flavin (BLUF) domain. These domains, one of the most studied types of biological photoreceptor, respond to blue light and either regulate the activity of an attached enzyme domain or change its affinity for a repressor protein. BLUF domains were discovered through studies of photo-induced movements of Euglena gracilis, a unicellular flagellate, and gene expression in the purple bacterium Rhodobacter sphaeroides, but the precise details of light activation remain unknown. Here, we describe crystal structures and the light regulation mechanism of the previously undescribed OaPAC, showing a central coiled coil transmits changes from the light-sensing domains to the active sites with minimal structural rearrangement. Site-directed mutants show residues essential for signal transduction over 45 Å across the protein. The use of the protein in living human cells is demonstrated with cAMP-dependent luciferase, showing a rapid and stable response to light over many hours and activation cycles. The structures determined in this study will assist future efforts to create artificial light-regulated control modules as part of a general optogenetic toolkit.

Abstract: Intracellular membrane trafficking, which is involved in diverse cellular processes, is dynamic and difficult to study in a spatiotemporal manner. Here we report an optogenetic strategy, termed light-activated reversible inhibition by assembled trap of intracellular membranes (IM-LARIAT), that uses various Rab GTPases combined with blue-light-induced hetero-interaction between cryptochrome 2 and CIB1. In this system, illumination induces a rapid and reversible intracellular membrane aggregation that disrupts the dynamics and functions of the targeted membrane. We applied IM-LARIAT to specifically perturb several Rab-mediated trafficking processes, including receptor transport, protein sorting and secretion, and signaling initiated from endosomes. We finally used this tool to reveal different functions of local Rab5-mediated and Rab11-mediated membrane trafficking in growth cones and soma of young hippocampal neurons. Our results show that IM-LARIAT is a versatile tool that can be used to dissect spatiotemporal functions of intracellular membranes in diverse systems.

Abstract: Blastocladiomycota fungi form motile zoospores that are guided by sensory photoreceptors to areas of optimal light conditions. We showed that the microbial rhodopsin of Blastocladiella emersonii is a rhodopsin-guanylyl cyclase (RhGC), a member of a previously uncharacterized rhodopsin class of light-activated enzymes that generate the second messenger cyclic guanosine monophosphate (cGMP). Upon application of a short light flash, recombinant RhGC converted within 8 ms into a signaling state with blue-shifted absorption from which the dark state recovered within 100 ms. When expressed in Xenopus oocytes, Chinese hamster ovary cells, or mammalian neurons, RhGC generated cGMP in response to green light in a light dose-dependent manner on a subsecond time scale. Thus, we propose RhGC as a versatile tool for the optogenetic analysis of cGMP-dependent signaling processes in cell biology and the neurosciences.

Abstract: Proper positioning of organelles by cytoskeleton-based motor proteins underlies cellular events such as signalling, polarization and growth. For many organelles, however, the precise connection between position and function has remained unclear, because strategies to control intracellular organelle positioning with spatiotemporal precision are lacking. Here we establish optical control of intracellular transport by using light-sensitive heterodimerization to recruit specific cytoskeletal motor proteins (kinesin, dynein or myosin) to selected cargoes. We demonstrate that the motility of peroxisomes, recycling endosomes and mitochondria can be locally and repeatedly induced or stopped, allowing rapid organelle repositioning. We applied this approach in primary rat hippocampal neurons to test how local positioning of recycling endosomes contributes to axon outgrowth and found that dynein-driven removal of endosomes from axonal growth cones reversibly suppressed axon growth, whereas kinesin-driven endosome enrichment enhanced growth. Our strategy for optogenetic control of organelle positioning will be widely applicable to explore site-specific organelle functions in different model systems.

Abstract: Receptor tyrosine kinases (RTKs) are a family of cell-surface receptors that have a key role in regulating critical cellular processes. Here, to understand and precisely control RTK signalling, we report the development of a genetically encoded, photoactivatable Trk (tropomyosin-related kinase) family of RTKs using a light-responsive module based on Arabidopsis thaliana cryptochrome 2. Blue-light stimulation (488 nm) of mammalian cells harbouring these receptors robustly upregulates canonical Trk signalling. A single light stimulus triggers transient signalling activation, which is reversibly tuned by repetitive delivery of blue-light pulses. In addition, the light-provoked process is induced in a spatially restricted and cell-specific manner. A prolonged patterned illumination causes sustained activation of extracellular signal-regulated kinase and promotes neurite outgrowth in a neuronal cell line, and induces filopodia formation in rat hippocampal neurons. These light-controllable receptors are expected to create experimental opportunities to spatiotemporally manipulate many biological processes both in vitro and in vivo.

Abstract: Dendritic spines are the primary sites of excitatory synaptic transmission in the vertebrate brain, and the morphology of these actin-rich structures correlates with synaptic function. Here we demonstrate a unique method for inducing spine enlargement and synaptic potentiation in dispersed hippocampal neurons, and use this technique to identify a coordinator of these processes; Ras-specific guanine nucleotide releasing factor 2 (RasGRF2). RasGRF2 is a dual Ras/Rac guanine nucleotide exchange factor (GEF) that is known to be necessary for long-term potentiation in situ. Contrary to the prevailing assumption, we find RasGRF2's Rac-GEF activity to be essential for synaptic potentiation by using a molecular replacement strategy designed to dissociate Rac- from Ras-GEF activities. Furthermore, we demonstrate that Rac1 activity itself is sufficient to rapidly modulate postsynaptic strength by using a photoactivatable derivative of this small GTPase. Because Rac1 is a major actin regulator, our results support a model where the initial phase of long-term potentiation is driven by the cytoskeleton.