Integration of light-controlled neuronal firing and fast circuit imaging

Spatio-temporal activation patterns of neuronal population evoked by optostimulation and the comparison to electrical microstimulation

Optostimulation and electrical microstimulation are well-established techniques that enable to artificially stimulate the brain. While the activation patterns evoked by microstimulation in cortical network are well characterized, much less is known for optostimulation. Specifically, the activation maps of neuronal population at the membrane potential level and direct measurements of these maps were barely reported. In addition, only a few studies compared the activation patterns evoked by microstimulation and optostimulation. In this study we addressed these issues by applying optostimulation in the barrel cortex of anesthetized rats after a short (ShortExp) or a long (LongExp) opsin expression time and compared it to microstimulation. We measured the membrane potential of neuronal populations at high spatial (meso-scale) and temporal resolution using voltage-sensitive dye imaging. Longer optostimulation pulses evoked higher neural responses spreading over larger region relative to short pulses. Interestingly, similar optostimulation pulses evoked stronger and more prolonged population response in the LongExp vs. the ShortExp condition. Finally, the spatial activation patterns evoked in the LongExp condition showed an intermediate state, with higher resemblance to the microstimulation at the stimulation site. Therefore, short microstimulation and optostimulation can induce wide spread activation, however the effects of optostimulation depend on the opsin expression time.

Optogenetic control of mesenchymal cell fate towards precise bone regeneration

Rationale: Spatial-temporal control of cell fate in vivo is of great importance for regenerative medicine. Currently, there remain no practical strategies to tune cell-fate spatial-temporally. Optogenetics is a biological technique that widely used to control cell activity in genetically defined neurons in a spatiotemporal-specific manner by light. In this study, optogenetics was repurposed for precise bone tissue regeneration.

Methods: Lhx8 and BMP2 genes, which are considered as the master genes for mesenchymal stem cell proliferation and differentiation respectively, were recombined into a customized optogenetic control system. In the system, Lhx8 was constitutively expressed, while BMP2 together with shLhx8 expression was driven by blue light.

Results: As expected, blue light induced BMP2 expression and inactivated Lhx8 expression in cells infected with the optogenetic control system. Optogenetic control of BMP2 and Lhx8 expression inversely regulates MSC fate in vitro. By animal study, we found that blue light could fine-tune the regeneration in vivo. Blue light illumination significantly promotes bone regeneration when the scaffold was loaded with MSCs infected with adeno-Lhx8, GI-Gal4DBD, LOV-VP16, and BMP2-shLhx8.

Conclusions: Together, our study revealed that optogenetic control of the master genes for mesenchymal stem cell proliferation and differentiation would be such a candidate strategy for precise regenerative medicine.

Long-term all-optical interrogation of cortical neurons in awake-behaving nonhuman primates

Whereas optogenetic techniques have proven successful in their ability to manipulate neuronal populations-with high spatial and temporal fidelity-in species ranging from insects to rodents, significant obstacles remain in their application to nonhuman primates (NHPs). Robust optogenetics-activated behavior and long-term monitoring of target neurons have been challenging in NHPs. Here, we present a method for all-optical interrogation (AOI), integrating optical stimulation and simultaneous two-photon (2P) imaging of neuronal populations in the primary visual cortex (V1) of awake rhesus macaques. A red-shifted channel-rhodopsin transgene (ChR1/VChR1 [C1V1]) and genetically encoded calcium indicators (genetically encoded calmodulin protein [GCaMP]5 or GCaMP6s) were delivered by adeno-associated viruses (AAVs) and subsequently expressed in V1 neuronal populations for months. We achieved optogenetic stimulation using both single-photon (1P) activation of neuronal populations and 2P activation of single cells, while simultaneously recording 2P calcium imaging in awake NHPs. Optogenetic manipulations of V1 neuronal populations produced reliable artificial visual percepts. Together, our advances show the feasibility of precise and stable AOI of cortical neurons in awake NHPs, which may lead to broad applications in high-level cognition and preclinical testing studies.

All-Optical Interrogation of Neural Circuits

There have been two recent revolutionary advances in neuroscience: First, genetically encoded activity sensors have brought the goal of optical detection of single action potentials in vivo within reach. Second, optogenetic actuators now allow the activity of neurons to be controlled with millisecond precision. These revolutions have now been combined, together with advanced microscopies, to allow "all-optical" readout and manipulation of activity in neural circuits with single-spike and single-neuron precision. This is a transformational advance that will open new frontiers in neuroscience research. Harnessing the power of light in the all-optical approach requires coexpression of genetically encoded activity sensors and optogenetic probes in the same neurons, as well as the ability to simultaneously target and record the light from the selected neurons. It has recently become possible to combine sensors and optical strategies that are sufficiently sensitive and cross talk free to enable single-action-potential sensitivity and precision for both readout and manipulation in the intact brain. The combination of simultaneous readout and manipulation from the same genetically defined cells will enable a wide range of new experiments as well as inspire new technologies for interacting with the brain. The advances described in this review herald a future where the traditional tools used for generations by physiologists to study and interact with the brain-stimulation and recording electrodes-can largely be replaced by light. We outline potential future developments in this field and discuss how the all-optical strategy can be applied to solve fundamental problems in neuroscience.

SIGNIFICANCE STATEMENT

This review describes the nexus of dramatic recent developments in optogenetic probes, genetically encoded activity sensors, and novel microscopies, which together allow the activity of neural circuits to be recorded and manipulated entirely using light. The optical and protein engineering strategies that form the basis of this "all-optical" approach are now sufficiently advanced to enable single-neuron and single-action potential precision for simultaneous readout and manipulation from the same functionally defined neurons in the intact brain. These advances promise to illuminate many fundamental challenges in neuroscience, including transforming our search for the neural code and the links between neural circuit activity and behavior.

Closed-loop and activity-guided optogenetic control

Advances in optical manipulation and observation of neural activity have set the stage for widespread implementation of closed-loop and activity-guided optical control of neural circuit dynamics. Closing the loop optogenetically (i.e., basing optogenetic stimulation on simultaneously observed dynamics in a principled way) is a powerful strategy for causal investigation of neural circuitry. In particular, observing and feeding back the effects of circuit interventions on physiologically relevant timescales is valuable for directly testing whether inferred models of dynamics, connectivity, and causation are accurate in vivo. Here we highlight technical and theoretical foundations as well as recent advances and opportunities in this area, and we review in detail the known caveats and limitations of optogenetic experimentation in the context of addressing these challenges with closed-loop optogenetic control in behaving animals.

Functional optical probing of the hippocampal trisynaptic circuit in vitro: network dynamics, filter properties, and polysynaptic induction of CA1 LTP

Decades of brain research have identified various parallel loops linking the hippocampus with neocortical areas, enabling the acquisition of spatial and episodic memories. Especially the hippocampal trisynaptic circuit [entorhinal cortex layer II → dentate gyrus (DG) → cornu ammonis (CA)-3 → CA1] was studied in great detail because of its seemingly simple connectivity and characteristic structures that are experimentally well accessible. While numerous researchers focused on functional aspects, obtained from a limited number of cells in distinct hippocampal subregions, little is known about the neuronal network dynamics which drive information across multiple synapses for subsequent long-term storage. Fast voltage-sensitive dye imaging in vitro allows real-time recording of activity patterns in large/meso-scale neuronal networks with high spatial resolution. In this way, we recently found that entorhinal theta-frequency input to the DG most effectively passes filter mechanisms of the trisynaptic circuit network, generating activity waves which propagate across the entire DG-CA axis. These "trisynaptic circuit waves" involve high-frequency firing of CA3 pyramidal neurons, leading to a rapid induction of classical NMDA receptor-dependent long-term potentiation (LTP) at CA3-CA1 synapses (CA1 LTP). CA1 LTP has been substantially evidenced to be essential for some forms of explicit learning in mammals. Here, we review data with particular reference to whole network-level approaches, illustrating how activity propagation can take place within the trisynaptic circuit to drive formation of CA1 LTP.

Optogenetic approaches for functional mouse brain mapping

To better understand the connectivity of the brain, it is important to map both structural and functional connections between neurons and cortical regions. In recent years, a set of optogenetic tools have been developed that permit selective manipulation and investigation of neural systems. These tools have enabled the mapping of functional connections between stimulated cortical targets and other brain regions. Advantages of the approach include the ability to arbitrarily stimulate brain regions that express opsins, allowing for brain mapping independent of behavior or sensory processing. The ability of opsins to be rapidly and locally activated allows for investigation of connectivity with spatial resolution on the order of single neurons and temporal resolution on the order of milliseconds. Optogenetic methods for functional mapping have been applied in experiments ranging from in vitro investigation of microcircuits, to in vivo probing of inter-regional cortical connections, to examination of global connections within the whole brain. We review recently developed functional mapping methods that use optogenetic single-point stimulation in the rodent brain and employ cellular electrophysiology, evoked motor movements, voltage sensitive dyes (VSDs), calcium indicators, or functional magnetic resonance imaging (fMRI) to assess activity. In particular we highlight results using red-shifted organic VSDs that permit high temporal resolution imaging in a manner spectrally separated from Channelrhodopsin-2 (ChR2) activation. VSD maps stimulated by ChR2 were dependent on intracortical synaptic activity and were able to reflect circuits used for sensory processing. Although the methods reviewed are powerful, challenges remain with respect to finding approaches that permit selective high temporal resolution assessment of stimulated activity in animals that can be followed longitudinally.

Development of transgenic animals for optogenetic manipulation of mammalian nervous system function: progress and prospects for behavioral neuroscience

Here we review the rapidly growing toolbox of transgenic mice and rats that exhibit functional expression of engineered opsins for neuronal activation and silencing with light. Collectively, these transgenic animals are enabling neuroscientists to access and manipulate the many diverse cell types in the mammalian nervous system in order to probe synaptic and circuitry connectivity, function, and dysfunction. The availability of transgenic lines affords important advantages such as stable and heritable transgene expression patterns across experimental cohorts. As such, the use of transgenic lines precludes the need for other costly and labor-intensive procedures to achieve functional transgene expression in each individual experimental animal. This represents an important consideration when large cohorts of experimental animals are desirable as in many common behavioral assays. We describe the diverse strategies that have been implemented for developing transgenic mouse and rat lines and highlight recent advances that have led to dramatic improvements in achieving functional transgene expression of engineered opsins. Furthermore, we discuss considerations and caveats associated with implementing recently developed transgenic lines for optogenetics-based experimentation. Lastly, we propose strategies that can be implemented to develop and refine the next generation of genetically modified animals for behaviorally-focused optogenetics-based applications.

Fast synaptic inhibition in spinal sensory processing and pain control

The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

Neurotransmitter phenotype plasticity: an unexpected mechanism in the toolbox of network activity homeostasis

The transmitter phenotype of a neuron has long been thought to be stable for the lifespan. Much as eyes have one color and do not change it over time, neurons have been thought to have one neurotransmitter and retain it for their lifetime. Both principles, exclusivity and stability, are challenged by recent data. More and more neurons in different regions of the brain appear to coexpress two or more neurotransmitters. Moreover, the profile of neurotransmitter expression of a given neuron has been shown to change over time, both during development and in response to changes in activity. The present review summarizes recent studies of this neurotransmitter phenotype plasticity (NPP). Homeostatic mechanisms of plasticity are aimed at maintaining the system within a functional range. They appear to be critical for optimal network operations and have been thought to operate largely by regulating intrinsic excitability, synapse number and synaptic strength. NPP provides a new and unexpected level of regulation of network homeostasis. We propose that it provides the basis for NT coexpression and discuss emerging issues and new questions for further studies in coming years.

The optogenetic (r)evolution

Optogenetics is a rapidly evolving field of technology that allows optical control of genetically targeted biological systems at high temporal and spatial resolution. By heterologous expression of light-sensitive microbial membrane proteins, opsins, cell type-specific depolarization or silencing can be optically induced on a millisecond time scale. What started in a petri dish is applicable today to more complex systems, ranging from the dissection of brain circuitries in vitro to behavioral analyses in freely moving animals. Persistent technical improvement has focused on the identification of new opsins, suitable for optogenetic purposes and genetic engineering of existing ones. Optical stimulation can be combined with various readouts defined by the desired resolution of the experimental setup. Although recent developments in optogenetics have largely focused on neuroscience it has lately been extended to other targets, including stem cell research and regenerative medicine. Further development of optogenetic approaches will not only highly increase our insight into health and disease states but might also pave the way for a future use in therapeutic applications.

The alarm response in zebrafish: innate fear in a vertebrate genetic model

The alarm response is an antipredator behavior displayed by many fish species and was first described 70 years ago. It is triggered through the olfactory system by substances released from injured skin and is characterized by dramatic, measurable changes in locomotion as well as physiology. We propose that this is an ideal time to revisit this response and to utilize it as an assay for understanding how neural circuits mediate innate fear. A suitable organism for these studies is the zebrafish, a genetic model with a rapidly expanding toolkit for molecular manipulation of the nervous system. Individual neurons mediating the response, ranging from receptor neurons to those in higher brain centers, should first be identified. New tools, specifically transgenic lines that allow spatial and temporal control of neural activity, provide a way to define and test the role of specific neurons, while genetic screens provide a route to identifying individual molecules essential for a normal response. Optical recording, which has proven successful in studies of information processing in the bulb, will provide valuable insights into neural circuitry function during the alarm response. When carried out on mutants, physiological analysis can provide insight into aspects of signal processing that are essential for normal behavior. The alarm response thus provides a paradigm to examine innate fear in a vertebrate system, enabling analysis at multiple levels from genes to the entire neural circuit. Additionally, the context dependency of the response can be utilized to investigate attention and decision making.

Shining light into the black box of spinal locomotor networks

Rhythmic activity is responsible for numerous essential motor functions including locomotion, breathing and chewing. In the case of locomotion, it has been realized for some time that the spinal cord contains sufficient circuitry to produce a sophisticated stepping pattern. However, the central pattern generator for locomotion in mammals has remained a 'black box' where inputs to the network were manipulated and the outputs interpreted. Over the last decade, new genetic approaches and techniques have been developed that provide ways to identify and manipulate the activity of classes of interneurons. The use of these techniques will be critically discussed and related to current models of network function.

Sparse and powerful cortical spikes

Activity in cortical networks is heterogeneous, sparse and often precisely timed. The functional significance of sparseness and precise spike timing is debated, but our understanding of the developmental and synaptic mechanisms that shape neuronal discharge patterns has improved. Evidence for highly specialized, selective and abstract cortical response properties is accumulating. Singe-cell stimulation experiments demonstrate a high sensitivity of cortical networks to the action potentials of some, but not all, single neurons. It is unclear how this sensitivity of cortical networks to small perturbations comes about and whether it is a generic property of cortex. The unforeseen sensitivity to cortical spikes puts serious constraints on the nature of neural coding schemes.

Targeting ligand-gated ion channels in neurology and psychiatry: is pharmacological promiscuity an obstacle or an opportunity?

BACKGROUND

The traditional emphasis on developing high specificity pharmaceuticals ("magic bullets") for the treatment of Neurological and Psychiatric disorders is being challenged by emerging pathophysiology concepts that view disease states as abnormal interactions within complex networks of molecular and cellular components. So-called network pharmacology focuses on modifying the behavior of entire systems rather than individual components, a therapeutic strategy that would ideally employ single pharmacological agents capable of interacting with multiple targets ("magic shotguns"). For this approach to be successful, however, a framework for understanding pharmacological "promiscuity"--the ability of individual agents to modulate multiple molecular targets--is needed.

PRESENTATION OF THE HYPOTHESIS

Pharmacological promiscuity is more often the rule than the exception for drugs that target the central nervous system (CNS). We hypothesize that promiscuity is an important contributor to clinical efficacy. Modulation patterns of existing therapeutic agents may provide critical templates for future drug discovery in Neurology and Psychiatry.

TESTING THE HYPOTHESIS

To demonstrate the extent of pharmacological promiscuity and develop a framework for guiding drug screening, we reviewed the ability of 170 therapeutic agents and endogenous molecules to directly modulate neurotransmitter receptors, a class of historically attractive therapeutic targets in Neurology and Psychiatry. The results are summarized in the form of 1) receptor-centric maps that illustrate the degree of promiscuity for GABA-, glycine-, serotonin-, and acetylcholine-gated ion channels, and 2) drug-centric maps that illustrated how characterization of promiscuity can guide drug development.

IMPLICATIONS OF THE HYPOTHESIS

Developing promiscuity maps of approved neuro-pharmaceuticals will provide therapeutic class-based templates against which candidate compounds can be screened. Importantly, compounds previously rejected in traditional screens due to poor specificity could be reconsidered in this framework. Further testing will require high throughput assays to systematically characterize interactions between available CNS-active drugs and surface receptors, both ionotropic and metabotropic.

Optogenetic interrogation of neural circuits: technology for probing mammalian brain structures

Elucidation of the neural substrates underlying complex animal behaviors depends on precise activity control tools, as well as compatible readout methods. Recent developments in optogenetics have addressed this need, opening up new possibilities for systems neuroscience. Interrogation of even deep neural circuits can be conducted by directly probing the necessity and sufficiency of defined circuit elements with millisecond-scale, cell type-specific optical perturbations, coupled with suitable readouts such as electrophysiology, optical circuit dynamics measures and freely moving behavior in mammals. Here we collect in detail our strategies for delivering microbial opsin genes to deep mammalian brain structures in vivo, along with protocols for integrating the resulting optical control with compatible readouts (electrophysiological, optical and behavioral). The procedures described here, from initial virus preparation to systems-level functional readout, can be completed within 4-5 weeks. Together, these methods may help in providing circuit-level insight into the dynamics underlying complex mammalian behaviors in health and disease.

Mapping cell fate and function using recombinase-based intersectional strategies

Cell types are typically defined by expression of a unique combination of genes, rather than a single gene. Intersectional methods therefore become crucial to selectively access these cells for higher resolution fate mapping and functional manipulations. Here, we discuss one such intersectional method. Two recombinase systems (Cre/loxP and Flp/FRT) work together to remove a double STOP cassette and thereby activate expression of a target transgene solely in cells defined by a particular pairwise combination of driver genes. Depending on the nature of the target transgene, this strategy can be used to deliver cell-lineage tracers, sensors, and/or effector molecules to highly selective cell types in vivo. In this chapter, we discuss concepts, reagents, and methods underlying this intersectional approach and encourage consideration of various intersectional and binary methods for accessing uniquely defined cell subsets in the mouse.

The Use of Lentiviral Vectors and Cre/loxP to Investigate the Function of Genes in Complex Behaviors

The use of conventional knockout technologies has proved valuable for understanding the role of key genes and proteins in development, disease states, and complex behaviors. However, these strategies are limited in that they produce broad changes in gene function throughout the neuroaxis and do little to identify the effects of such changes on neural circuits thought to be involved in distinct functions. Because the molecular functions of genes often depend on the specific neuronal circuit in which they are expressed, restricting gene manipulation to specific brain regions and times may be more useful for understanding gene functions. Conditional gene manipulation strategies offer a powerful alternative. In this report we briefly describe two conditional gene strategies that are increasingly being used to investigate the role of genes in behavior – the Cre/loxP recombination system and lentiviral vectors. Next, we summarize a number of recent experiments which have used these techniques to investigate behavior after spatial and/or temporal and gene manipulation. These conditional gene targeting strategies provide useful tools to study the endogenous mechanisms underlying complex behaviors and to model disease states resulting from aberrant gene expression.

Complete optical neurophysiology: toward optical stimulation and recording of neural tissue

Direct optical methods to stimulate and record neural activity provide artifact-free, noninvasive, and noncontact neurophysiological procedures. For stimulation, focused mid-infrared light alters membrane potential and activates individual neural processes. Simultaneous intrinsic scattered light parameters, including birefringence changes, can record neural activity with signals similar to potentiometric dyes. The simultaneous combination of optical stimulation and optical recording techniques provide the potential for powerful tools that may someday remove the need for invasive wires during electrophysiological recordings.

Adult neurogenesis, mental health, and mental illness: hope or hype?

Psychiatric and neurologic disorders take an enormous toll on society. Alleviating the devastating symptoms and consequences of neuropsychiatric disorders such as addiction, depression, epilepsy, and schizophrenia is a main force driving clinical and basic researchers alike. By elucidating these disease neuromechanisms, researchers hope to better define treatments and preventive therapies. Research suggests that regulation of adult hippocampal neurogenesis represents a promising approach to treating and perhaps preventing mental illness. Here we appraise the role of adult hippocampal neurogenesis in major psychiatric and neurologic disorders within the essential framework of recent progress made in understanding "normal" adult neurogenesis. Topics addressed include the following: the life cycle of an adult hippocampal stem cell and the implications for aging; links between learning and hippocampal neurogenesis; the reciprocal relationship between cocaine self-administration and adult hippocampal neurogenesis; the role of adult neurogenesis in an animal model of depression and response to antidepressant exposure; the impact of neonatal seizures on dentate gyrus neurogenesis; and the contribution of a schizophrenia-susceptibility gene to adult hippocampal neurogenesis. These topics are discussed in light of the regulation of adult neurogenesis, the relationship to normal neurogenesis in adulthood and aging, and, importantly, the manipulation of neurogenesis to promote mental health and treat mental illness.

eNpHR: a Natronomonas halorhodopsin enhanced for optogenetic applications

Temporally precise inhibition of distinct cell types in the intact nervous system has been enabled by the microbial halorhodopsin NpHR, a fast light-activated electrogenic Cl(-) pump. While neurons can be optically hyperpolarized and inhibited from firing action potentials at moderate NpHR expression levels, we have encountered challenges with pushing expression to extremely high levels, including apparent intracellular accumulations. We therefore sought to molecularly engineer NpHR to achieve strong expression without these cellular side effects. We found that high expression correlated with endoplasmic reticulum (ER) accumulation, and that under these conditions NpHR colocalized with ER proteins containing the KDEL ER retention sequence. We screened a number of different putative modulators of membrane trafficking and identified a combination of two motifs, an N-terminal signal peptide and a C-terminal ER export sequence, that markedly promoted membrane localization and ER export defined by confocal microscopy and whole-cell patch clamp. The modified NpHR displayed increased peak photocurrent in the absence of aggregations or toxicity, and potent optical inhibition was observed not only in vitro but also in vivo with thalamic single-unit recording. The new enhanced NpHR (eNpHR) allows safe, high-level expression in mammalian neurons, without toxicity and with augmented inhibitory function, in vitro and in vivo.