Optogenetics - Bringing light into the darkness of mammalian signal transduction

Simultaneous spectral illumination of microplates for high-throughput optogenetics and photobiology

The biophysical characterization and engineering of optogenetic tools and photobiological systems has been hampered by the lack of efficient methods for spectral illumination of microplates for high-throughput analysis of action spectra. Current methods to determine action spectra only allow the sequential spectral illumination of individual wells. Here we present the open-source RainbowCap-system, which combines LEDs and optical filters in a standard 96-well microplate format for simultaneous and spectrally defined illumination. The RainbowCap provides equal photon flux for each wavelength, with the output of the LEDs narrowed by optical bandpass filters. We validated the RainbowCap for photoactivatable G protein-coupled receptors (opto-GPCRs) and enzymes for the control of intracellular downstream signaling. The simultaneous, spectrally defined illumination provides minimal interruption during time-series measurements, while resolving 10 nm differences in the action spectra of optogenetic proteins under identical experimental conditions. The RainbowCap is also suitable for studying the spectral dependence of light-regulated gene expression in bacteria, which requires illumination over several hours. In summary, the RainbowCap provides high-throughput spectral illumination of microplates, while its modular, customizable design allows easy adaptation to a wide range of optogenetic and photobiological applications.

Controlling the Potency of T Cell Activation Using an Optically Tunable Chimeric Antigen Receptor

The ability of biological systems to convert inputs from their environment into information to guide future decisions is central to life and a matter of great importance. While we know the components of many of the signaling networks that make these decisions, our understanding of the dynamic flow of information between these parts remains far more limited. T cells are an essential white blood cell type of an adaptive immune response and can discriminate between healthy and infected cells with remarkable sensitivity. This chapter describes the use of a synthetic T-cell receptor (OptoCAR) that is optically tunable within cell conjugates, providing control over the duration, and intensity of intracellular T-cell signaling dynamics. Optical control can also provide control over signaling with high spatial precision, and the OptoCAR is likely to find application more generally when modulating T-cell function with imaging approaches.

An optogenetic-phosphoproteomic study reveals dynamic Akt1 signaling profiles in endothelial cells

The serine/threonine kinase AKT is a central node in cell signaling. While aberrant AKT activation underlies the development of a variety of human diseases, how different patterns of AKT-dependent phosphorylation dictate downstream signaling and phenotypic outcomes remains largely enigmatic. Herein, we perform a systems-level analysis that integrates methodological advances in optogenetics, mass spectrometry-based phosphoproteomics, and bioinformatics to elucidate how different intensity, duration, and pattern of Akt1 stimulation lead to distinct temporal phosphorylation profiles in vascular endothelial cells. Through the analysis of ~35,000 phosphorylation sites across multiple conditions precisely controlled by light stimulation, we identify a series of signaling circuits activated downstream of Akt1 and interrogate how Akt1 signaling integrates with growth factor signaling in endothelial cells. Furthermore, our results categorize kinase substrates that are preferably activated by oscillating, transient, and sustained Akt1 signals. We validate a list of phosphorylation sites that covaried with Akt1 phosphorylation across experimental conditions as potential Akt1 substrates. Our resulting dataset provides a rich resource for future studies on AKT signaling and dynamics.

Optogenetic Methods in Plant Biology

Optogenetics is a technique employing natural or genetically engineered photoreceptors in transgene organisms to manipulate biological activities with light. Light can be turned on or off, and adjusting its intensity and duration allows optogenetic fine-tuning of cellular processes in a noninvasive and spatiotemporally resolved manner. Since the introduction of Channelrhodopsin-2 and phytochrome-based switches nearly 20 years ago, optogenetic tools have been applied in a variety of model organisms with enormous success, but rarely in plants. For a long time, the dependence of plant growth on light and the absence of retinal, the rhodopsin chromophore, prevented the establishment of plant optogenetics until recent progress overcame these difficulties. We summarize the recent results of work in the field to control plant growth and cellular motion via green light-gated ion channels and present successful applications to light-control gene expression with single or combined photoswitches in plants. Furthermore, we highlight the technical requirements and options for future plant optogenetic research.

Comparative and Temporal Characterization of LPS and Blue-Light-Induced TLR4 Signal Transduction and Gene Expression in Optogenetically Manipulated Endothelial Cells

In endothelial cells (ECs), stimulation of Toll-like receptor 4 (TLR4) by the endotoxin lipopolysaccharide (LPS) induces the release of diverse pro-inflammatory mediators, beneficial in controlling bacterial infections. However, their systemic secretion is a main driver of sepsis and chronic inflammatory diseases. Since distinct and rapid induction of TLR4 signaling is difficult to achieve with LPS due to the specific and non-specific affinity to other surface molecules and receptors, we engineered new light-oxygen-voltage-sensing (LOV)-domain-based optogenetic endothelial cell lines (opto-TLR4-LOV LECs and opto-TLR4-LOV HUVECs) that allow fast, precise temporal, and reversible activation of TLR4 signaling pathways. Using quantitative mass-spectrometry, RT-qPCR, and Western blot analysis, we show that pro-inflammatory proteins were not only expressed differently, but also had a different time course when the cells were stimulated with light or LPS. Additional functional assays demonstrated that light induction promoted chemotaxis of THP-1 cells, disruption of the EC monolayer and transmigration. In contrast, ECs incorporating a truncated version of the TLR4 extracellular domain (opto-TLR4 ΔECD2-LOV LECs) revealed high basal activity with fast depletion of the cell signaling system upon illumination. We conclude that the established optogenetic cell lines are well suited to induce rapid and precise photoactivation of TLR4, allowing receptor-specific studies.

Tailoring baker's yeast Saccharomyces cerevisiae for functional testing of channelrhodopsin

Channelrhodopsin 2 (ChR2) and its variants are the most frequent tools for remote manipulation of electrical properties in cells via light. Ongoing attempts try to enlarge their functional spectrum with respect to ion selectivity, light sensitivity and protein trafficking by mutations, protein engineering and environmental mining of ChR2 variants. A shortcoming in the required functional testing of large numbers of ChR2 variants is the lack of an easy screening system. Baker's yeast, which was successfully employed for testing ion channels from eukaryotes has not yet been used for screening of ChR2s, because they neither produce the retinal chromophore nor its precursor carotenoids. We found that addition of retinal to the external medium was not sufficient for detecting robust ChR activity in yeast in simple growth assays. This obstacle was overcome by metabolic engineering of a yeast strain, which constitutively produces retinal. In proof of concept experiments we functionally express different ChR variants in these cells and monitor their blue light induced activity in simple growth assays. We find that light activation of ChR augments an influx of Na+ with a consequent inhibition of cell growth. In a K+ uptake deficient yeast strain, growth can be rescued in selective medium by the blue light induced K+ conductance of ChR. This yeast strain can now be used as chassis for screening of new functional ChR variants and mutant libraries in simple yeast growth assays under defined selective conditions.

Biomolecular condensates in epithelial junctions

Epithelial junctions are transmembrane protein complexes that regulate cell adhesion, cell polarity, tissue permeability, and tissue mechanics. Most junctional complexes contain membrane attached cytoplasmic plaques that regulate junction assembly and are composed of multivalent scaffold proteins. In this review, we discuss phase separation of multivalent proteins as a general process that drives assembly of many membrane-less cellular compartments. And we summarise recent evidence that phase separation of junctional scaffold proteins is involved in the assembly of tight junctions and focal adhesions.

Optogenetic technologies in translational cancer research

Gene and cell therapies are widely recognized as future cancer therapeutics but poor controllability limits their clinical applications. Optogenetics, the use of light-controlled proteins to precisely spatiotemporally regulate the activity of genes and cells, opens up new possibilities for cancer treatment. Light of specific wavelength can activate the immune response, oncolytic activity and modulate cell signaling in tumor cells non-invasively, in dosed manner, with tissue confined action and without side effects of conventional therapies. Here, we review optogenetic approaches in cancer research, their clinical potential and challenges of incorporating optogenetics in cancer therapy. We critically discuss beneficial combinations of optogenetic technologies with therapeutic nanobodies, T-cell activation and CAR-T cell approaches, genome editors and oncolytic viruses. We consider viral vectors and nanoparticles for delivering optogenetic payloads and activating light to tumors. Finally, we highlight herein the prospects for integrating optogenetics into immunotherapy as a novel, fast, reversible and safe approach to cancer treatment.

Live cell microscopy: From image to insight

Live-cell microscopy is a powerful tool that can reveal cellular behavior as well as the underlying molecular processes. A key advantage of microscopy is that by visualizing biological processes, it can provide direct insights. Nevertheless, live-cell imaging can be technically challenging and prone to artifacts. For a successful experiment, many careful decisions are required at all steps from hardware selection to downstream image analysis. Facing these questions can be particularly intimidating due to the requirement for expertise in multiple disciplines, ranging from optics, biophysics, and programming to cell biology. In this review, we aim to summarize the key points that need to be considered when setting up and analyzing a live-cell imaging experiment. While we put a particular focus on yeast, many of the concepts discussed are applicable also to other organisms. In addition, we discuss reporting and data sharing strategies that we think are critical to improve reproducibility in the field.

Conformational alterations in unidirectional ion transport of a light-driven chloride pump revealed using X-ray free electron lasers

Light-driven chloride pumps have been identified in various species, including archaea and marine flavobacteria. The function of ion transportation controllable by light is utilized for optogenetics tools in neuroscience. Chloride pumps differ among species, in terms of amino acid homology and structural similarity. Our time-resolved crystallographic studies using X-ray free electron lasers reveal the molecular mechanism of halide ion transfer in a light-driven chloride pump from a marine flavobacterium. Our data indicate a common mechanism in chloride pumping rhodopsins, as compared to previous low-temperature trapping studies of chloride pumps. These findings are significant not only for further improvements of optogenetic tools but also for a general understanding of the ion pumping mechanisms of microbial rhodopsins.

Light-driven chloride-pumping rhodopsins actively transport anions, including various halide ions, across cell membranes. Recent studies using time-resolved serial femtosecond crystallography (TR-SFX) have uncovered the structural changes and ion transfer mechanisms in light-driven cation-pumping rhodopsins. However, the mechanism by which the conformational changes pump an anion to achieve unidirectional ion transport, from the extracellular side to the cytoplasmic side, in anion-pumping rhodopsins remains enigmatic. We have collected TR-SFX data of Nonlabens marinus rhodopsin-3 (NM-R3), derived from a marine flavobacterium, at 10-µs and 1-ms time points after photoexcitation. Our structural analysis reveals the conformational alterations during ion transfer and after ion release. Movements of the retinal chromophore initially displace a conserved tryptophan to the cytoplasmic side of NM-R3, accompanied by a slight shift of the halide ion bound to the retinal. After ion release, the inward movements of helix C and helix G and the lateral displacements of the retinal block access to the extracellular side of NM-R3. Anomalous signal data have also been obtained from NM-R3 crystals containing iodide ions. The anomalous density maps provide insight into the halide binding site for ion transfer in NM-R3.

Molecular Biology of Microbial Rhodopsins

Research on type 1 rhodopsins spans now a history of 50 years. Originally, just archaeal ion pumps and sensors have been discovered. However, with modern genetic techniques and gene sequencing tools, more and more proteins were identified in all kingdoms of life. Spectroscopic and other biophysical studies revealed quite diverse functions. Ion pumps, sensors, and channels are imprinted in the same seven-helix transmembrane protein scaffold carrying a retinal prosthetic group. In this review, molecular biology methods are described, which enabled the elucidation of their function and structure leading to optogenetic applications.

Methods for assessment of membrane protrusion dynamics

Membrane protrusions are a critical facet of cell function. Mediating fundamental processes such as cell migration, cell-cell interactions, phagocytosis, as well as assessment and remodeling of the cell environment. Different protrusion types and morphologies can promote different cellular functions and occur downstream of distinct signaling pathways. As such, techniques to quantify and understand the inner workings of protrusion dynamics are critical for a comprehensive understanding of cell biology. In this chapter, we describe approaches to analyze cellular protrusions and correlate physical changes in cell morphology with biochemical signaling processes. We address methods to quantify and characterize protrusion types and velocity, mathematical approaches to predictive models of cytoskeletal changes, and implementation of protein engineering and biosensor design to dissect cell signaling driving protrusive activity. Combining these approaches allows cell biologists to develop a comprehensive understanding of the dynamics of membrane protrusions.

Optogenetics in bacteria - applications and opportunities

Optogenetics holds the promise of controlling biological processes with superb temporal and spatial resolution at minimal perturbation. Although many of the light-reactive proteins used in optogenetic systems are derived from prokaryotes, applications were largely limited to eukaryotes for a long time. In recent years, however, an increasing number of microbiologists use optogenetics as a powerful new tool to study and control key aspects of bacterial biology in a fast and often reversible manner. After a brief discussion of optogenetic principles, this review provides an overview of the rapidly growing number of optogenetic applications in bacteria, with a particular focus on studies venturing beyond transcriptional control. To guide future experiments, we highlight helpful tools, provide considerations for successful application of optogenetics in bacterial systems, and identify particular opportunities and challenges that arise when applying these approaches in bacteria.

The state of the art of biomedical applications of optogenetics

BACKGROUND AND OBJECTIVE

Optogenetics has opened new insights into biomedical research with the ability to manipulate and control cellular activity using light in combination with genetically engineered photosensitive proteins. By stimulating with light, this method provides high spatiotemporal and high specificity resolution, which is in contrast to conventional pharmacological or electrical stimulation. Optogenetics was initially introduced to control neural activities but was gradually extended to other biomedical fields.

STUDY DESIGN

In this paper, firstly, we summarize the current optogenetic tools stimulated by different light sources, including lasers, light-emitting diodes, and laser diodes. Second, we outline the variety of biomedical applications of optogenetics not only for neuronal circuits but also for various kinds of cells and tissues from cardiomyocytes to ganglion cells. Furthermore, we highlight the potential of this technique for treating neurological disorders, cardiac arrhythmia, visual impairment, hearing loss, and urinary bladder diseases as well as clarify the mechanisms underlying cancer progression and control of stem cell differentiation.

CONCLUSION

We sought to summarize the various types of promising applications of optogenetics to treat a broad spectrum of disorders. It is conceivable to expect that optogenetics profits a growing number of patients suffering from a range of different diseases in the near future.

A Novel Research Technology to Explore the Mystery of Traditional Chinese Medicine: Optogenetics

Traditional Chinese medicine (TCM) is gaining increasing popularity worldwide for the function of health promotion and adjuvant therapy. However, the world's understanding of TCM is far from enough, which seriously limits the modernization and internationalization of TCM. Therefore, modern and efficient analytical methods are urgently needed to understand the mechanism of TCM. Optogenetics is one of the most prevalent technologies in the 21st century and has been used to explore life science, especially neuroscience. It already has had great influences in the study of neural circuits and animal models of mental diseases and was named “Method of the Year” by the Nature Methods journal in 2010. Increased interests occurred in the applications of optogenetics to explore a myriad of medical and mental health disorders. However, it has not so far been noticed by TCM researchers. We elaborated on an idea that introducing this technique into the field of TCM research to improve diagnosis, treatments, and evaluating the therapeutic effects. In this review, we made a systematic prospect in the theory, feasibility, and application of TCM optogenetics. We mainly focused on applying optogenetic methodologies to make a more comprehensive understanding of TCM.

Optogenetically-induced multimerization of the dopamine transporter increases uptake and trafficking to the plasma membrane

The dopamine transporter (DAT) is essential for the reuptake of the released neurotransmitter dopamine (DA) in the brain. Psychostimulants, methamphetamine and cocaine, have been reported to induce the formation of DAT multimeric complexes in vivo and in vitro. The interpretation of DAT multimer function has been primarily in the context of compounds that induce structural and functional modifications of the DAT, complicating the understanding of the significance of DAT multimers. To examine multimerization in the absence of DAT ligands as well as in their presence, we developed a novel, optogenetic fusion chimera of cryptochrome 2 and DAT with an mCherry fluorescent reporter (Cry2-DAT). Using blue light to induce Cry2-DAT multimeric protein complex formation, we were able to simultaneously test the functional contributions of DAT multimerization in the absence or presence of substrates or inhibitors with high spatiotemporal precision. We found that blue light-stimulated Cry2-DAT multimers significantly increased IDT307 uptake and MFZ 9-18 binding in the absence of ligands as well as after methamphetamine and nomifensine treatment. Blue light-induced Cry2-DAT multimerization increased colocalization with recycling endosomal marker Rab11 and had decreased presence in Rab5-positive early endosomes and Rab7-positive late endosomes. Our data suggest that the increased uptake and binding results from induced and rapid trafficking of DAT multimers to the plasma membrane. Our data suggest that DAT multimers may function to help maintain DA homeostasis.

Multichromatic Control of Signaling Pathways in Mammalian Cells

The precise control of signaling proteins is a prerequisite to decipher the complexity of the signaling network and to reveal and to study pathways involved in regulating cellular metabolism and gene expression. Optogenetic approaches play an emerging role as they enable the spatiotemporal control of signaling processes. Herein, a multichromatic system is developed by combining the blue light cryptochrome 2 system and the red/far-red light phytochrome B system. The use of three wavelengths allows the orthogonal control of the RAF/ERK and the AKT signaling pathway. Continuous exposure of cells to blue light leads to activation of AKT while simultaneous pulses of red and far-red light enable the modulation of ERK signaling in cells with constantly active AKT signaling. The optimized, orthogonal multichromatic system presented here is a valuable tool to better understand the fine grained and intricate processes involved in cell fate decisions.

Non-Cell-Autonomous Mechanisms in Radial Projection Neuron Migration in the Developing Cerebral Cortex

Concerted radial migration of newly born cortical projection neurons, from their birthplace to their final target lamina, is a key step in the assembly of the cerebral cortex. The cellular and molecular mechanisms regulating the specific sequential steps of radial neuronal migration in vivo are however still unclear, let alone the effects and interactions with the extracellular environment. In any in vivo context, cells will always be exposed to a complex extracellular environment consisting of (1) secreted factors acting as potential signaling cues, (2) the extracellular matrix, and (3) other cells providing cell–cell interaction through receptors and/or direct physical stimuli. Most studies so far have described and focused mainly on intrinsic cell-autonomous gene functions in neuronal migration but there is accumulating evidence that non-cell-autonomous-, local-, systemic-, and/or whole tissue-wide effects substantially contribute to the regulation of radial neuronal migration. These non-cell-autonomous effects may differentially affect cortical neuron migration in distinct cellular environments. However, the cellular and molecular natures of such non-cell-autonomous mechanisms are mostly unknown. Furthermore, physical forces due to collective migration and/or community effects (i.e., interactions with surrounding cells) may play important roles in neocortical projection neuron migration. In this concise review, we first outline distinct models of non-cell-autonomous interactions of cortical projection neurons along their radial migration trajectory during development. We then summarize experimental assays and platforms that can be utilized to visualize and potentially probe non-cell-autonomous mechanisms. Lastly, we define key questions to address in the future.

Lights up on organelles: Optogenetic tools to control subcellular structure and organization

Since the neurobiological inception of optogenetics, light-controlled molecular perturbations have been applied in many scientific disciplines to both manipulate and observe cellular function. Proteins exhibiting light-sensitive conformational changes provide researchers with avenues for spatiotemporal control over the cellular environment and serve as valuable alternatives to chemically inducible systems. Optogenetic approaches have been developed to target proteins to specific subcellular compartments, allowing for the manipulation of nuclear translocation and plasma membrane morphology. Additionally, these tools have been harnessed for molecular interrogation of organelle function, location, and dynamics. Optogenetic approaches offer novel ways to answer fundamental biological questions and to improve the efficiency of bioengineered cell factories by controlling the assembly of synthetic organelles. This review first provides a summary of available optogenetic systems with an emphasis on their organelle-specific utility. It then explores the strategies employed for organelle targeting and concludes by discussing our perspective on the future of optogenetics to control subcellular structure and organization. This article is categorized under: Metabolic Diseases > Molecular and Cellular Physiology.

ERK signalling: a master regulator of cell behaviour, life and fate

The proteins extracellular signal-regulated kinase 1 (ERK1) and ERK2 are the downstream components of a phosphorelay pathway that conveys growth and mitogenic signals largely channelled by the small RAS GTPases. By phosphorylating widely diverse substrates, ERK proteins govern a variety of evolutionarily conserved cellular processes in metazoans, the dysregulation of which contributes to the cause of distinct human diseases. The mechanisms underlying the regulation of ERK1 and ERK2, their mode of action and their impact on the development and homeostasis of various organisms have been the focus of much attention for nearly three decades. In this Review, we discuss the current understanding of this important class of kinases. We begin with a brief overview of the structure, regulation, substrate recognition and subcellular localization of ERK1 and ERK2. We then systematically discuss how ERK signalling regulates six fundamental cellular processes in response to extracellular cues. These processes are cell proliferation, cell survival, cell growth, cell metabolism, cell migration and cell differentiation.

OpEn-Tag-A Customizable Optogenetic Toolbox To Dissect Subcellular Signaling

Subcellular localization of signal molecules is a hallmark in organizing the signaling network. OpEn-Tag is a modular optogenetic endomembrane targeting toolbox that allows alteration of the localization and therefore the activity of signaling processes with the spatiotemporal resolution of optogenetics. OpEn-Tag is a two-component system employing (1) a variety of targeting peptides fused to and thereby dictating the localization of mCherry-labeled cryptochrome 2 binding protein CIBN toward distinct endomembranes and (2) the cytosolic, fluorescence-labeled blue light photoreceptor cryptochrome 2 as a customizable building block that can be fused to proteins of interest. The combination of OpEn-Tag with growth factor stimulation or the use of two membrane anchor sequences allows investigation of multilayered signal transduction processes as demonstrated here for the protein kinase AKT.

Engineering Adenylate Cyclase Activated by Near-Infrared Window Light for Mammalian Optogenetic Applications

Light in the near-infrared optical window (NIRW) penetrates deep through mammalian tissues, including the skull and brain tissue. Here we engineered an adenylate cyclase (AC) activated by NIRW light (NIRW-AC) and suitable for mammalian applications. To accomplish this goal, we constructed fusions of several bacteriophytochrome photosensory and bacterial AC modules using guidelines for designing chimeric homodimeric bacteriophytochromes. One engineered NIRW-AC, designated IlaM5, has significantly higher activity at 37 °C, is better expressed in mammalian cells, and can mediate cAMP-dependent photoactivation of gene expression in mammalian cells, in favorable contrast to the NIRW-ACs engineered earlier. The ilaM5 gene expressed from an AAV vector was delivered into the ventral basal thalamus region of the mouse brain, resulting in the light-controlled suppression of the cAMP-dependent wave pattern of the sleeping brain known as spindle oscillations. Reversible spindle oscillation suppression was observed in sleeping mice exposed to light from an external light source. This study confirms the robustness of principles of homodimeric bacteriophytochrome engineering, describes a NIRW-AC suitable for mammalian optogenetic applications, and demonstrates the feasibility of controlling brain activity via NIRW-ACs using transcranial irradiation.

Blue-Light Receptors for Optogenetics

Sensory photoreceptors underpin light-dependent adaptations of organismal physiology, development, and behavior in nature. Adapted for optogenetics, sensory photoreceptors become genetically encoded actuators and reporters to enable the noninvasive, spatiotemporally accurate and reversible control by light of cellular processes. Rooted in a mechanistic understanding of natural photoreceptors, artificial photoreceptors with customized light-gated function have been engineered that greatly expand the scope of optogenetics beyond the original application of light-controlled ion flow. As we survey presently, UV/blue-light-sensitive photoreceptors have particularly allowed optogenetics to transcend its initial neuroscience applications by unlocking numerous additional cellular processes and parameters for optogenetic intervention, including gene expression, DNA recombination, subcellular localization, cytoskeleton dynamics, intracellular protein stability, signal transduction cascades, apoptosis, and enzyme activity. The engineering of novel photoreceptors benefits from powerful and reusable design strategies, most importantly light-dependent protein association and (un)folding reactions. Additionally, modified versions of these same sensory photoreceptors serve as fluorescent proteins and generators of singlet oxygen, thereby further enriching the optogenetic toolkit. The available and upcoming UV/blue-light-sensitive actuators and reporters enable the detailed and quantitative interrogation of cellular signal networks and processes in increasingly more precise and illuminating manners.

Differential effects of transient and sustained activation of BDNF-TrkB signaling

Brain-derived neurotrophic factor (BDNF) serves a pleiotropic role in the central nervous system, ranging from promoting neuronal survival and differentiation during development and synaptic modulation in the adult. An important, yet unanswered question is how BDNF could serve such diverse functions, sometimes in the same cell. At least two modes of BDNF actions have been elucidated so far based on BDNF signaling kinetics and/or the activity status of the responding neurons. Acute and gradual increases in extracellular BDNF concentrations elicit, respectively, transient and sustained activation of TrkB receptor and its downstream signaling, leading to differential molecular and cellular functions. In cultured neurons, sustained TrkB activation promotes neuronal dendritic arborization and spinogenesis, whereas transient TrkB activation facilitates dendritic growth and spine morphogenesis. In hippocampal slices, slow delivery of BDNF facilitates LTP, whereas fast application of BDNF enhances basal synaptic transmission in schaffer collateral synapses. High-frequency stimulation of neurons converts BDNF-induced TrkB signaling from a transient to a sustained mode. These initial insights lay the foundation for future investigation of the BDNF-TrkB pathway, and analogous signaling pathways to gain a comprehensive understanding to enable translational research. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 647-659, 2018.