Overlapping representations of food and social stimuli in mouse VTA dopamine neurons.

Dopamine neurons of the ventral tegmental area (VTADA) respond to food and social stimuli and contribute to both forms of motivation. However, it is unclear whether the same or different VTADA neurons encode these different stimuli. To address this question, we performed two-photon calcium imaging in mice presented with food and conspecifics and found statistically significant overlap in the populations responsive to both stimuli. Both hunger and opposite-sex social experience further increased the proportion of neurons that respond to both stimuli, implying that increasing motivation for one stimulus increases overlap. In addition, single-nucleus RNA sequencing revealed significant co-expression of feeding- and social-hormone-related genes in individual VTADA neurons. Taken together, our functional and transcriptional data suggest overlapping VTADA populations underlie food and social motivation.

Overlapping representations of food and social stimuli in VTA dopamine neurons.

Dopamine neurons of the ventral tegmental area (VTA DA ) respond to food and social stimuli and contribute to both forms of motivation. However, it is unclear if the same or different VTA DA neurons encode these different stimuli. To address this question, we performed 2-photon calcium imaging in mice presented with food and conspecifics, and found statistically significant overlap in the populations responsive to both stimuli. Both hunger and opposite-sex social experience further increased the proportion of neurons that respond to both stimuli, implying that modifying motivation for one stimulus affects responses to both stimuli. In addition, single-nucleus RNA sequencing revealed significant co-expression of feeding- and social-hormone related genes in individual VTA DA neurons. Taken together, our functional and transcriptional data suggest overlapping VTA DA populations underlie food and social motivation.

BIFROST: a method for registering diverse imaging datasets.

Quantitative comparison of brain-wide neural dynamics across different experimental conditions often requires precise alignment to a common set of anatomical coordinates. While such approaches are routinely applied in functional magnetic resonance imaging (fMRI), registering in vivo fluorescence imaging data to ex vivo-derived reference atlases is challenging, given the many differences in imaging modality, microscope specification, and sample preparation. Moreover, in many systems, animal to animal variation in brain structure limits registration precision. Using the highly stereotyped architecture of the fruit fly brain as a model, we overcome these challenges by building a reference atlas based directly on in vivo multiphoton-imaged brains, called the Functional Drosophila Atlas (FDA). We then develop a novel two-step pipeline, BrIdge For Registering Over Statistical Templates (BIFROST), for transforming neural imaging data into this common space, and for importing ex vivo resources, such as connectomes. Using genetically labeled cell types to provide ground truth, we demonstrate that this method allows voxel registration with micron precision. Thus, this method provides a generalizable pipeline for registering neural activity datasets to one another, allowing quantitative comparisons across experiments, microscopes, genotypes, and anatomical atlases, including connectomes.

Corrigendum: An accumulation-of-evidence task using visual pulses for mice navigating in virtual reality.

[This corrects the article DOI: 10.3389/fnbeh.2018.00036.].

Sequential and efficient neural-population coding of complex task information.

Recent work has highlighted that many types of variables are represented in each neocortical area. How can these many neural representations be organized together without interference and coherently maintained/updated through time? We recorded from excitatory neural populations in posterior cortices as mice performed a complex, dynamic task involving multiple interrelated variables. The neural encoding implied that highly correlated task variables were represented by less-correlated neural population modes, while pairs of neurons exhibited a spectrum of signal correlations. This finding relates to principles of efficient coding, but notably utilizes neural population modes as the encoding unit and suggests partial whitening of task-specific information where different variables are represented with different signal-to-noise levels. Remarkably, this encoding function was multiplexed with sequential neural dynamics yet reliably followed changes in task-variable correlations throughout the trial. We suggest that neural circuits can implement time-dependent encodings in a simple way using random sequential dynamics as a temporal scaffold.

Auditory activity is diverse and widespread throughout the central brain of Drosophila.

Sensory pathways are typically studied by starting at receptor neurons and following postsynaptic neurons into the brain. However, this leads to a bias in analyses of activity toward the earliest layers of processing. Here, we present new methods for volumetric neural imaging with precise across-brain registration to characterize auditory activity throughout the entire central brain of Drosophila and make comparisons across trials, individuals and sexes. We discover that auditory activity is present in most central brain regions and in neurons responsive to other modalities. Auditory responses are temporally diverse, but the majority of activity is tuned to courtship song features. Auditory responses are stereotyped across trials and animals in early mechanosensory regions, becoming more variable at higher layers of the putative pathway, and this variability is largely independent of ongoing movements. This study highlights the power of using an unbiased, brain-wide approach for mapping the functional organization of sensory activity.

Shared Song Detector Neurons in Drosophila Male and Female Brains Drive Sex-Specific Behaviors.

Males and females often produce distinct responses to the same sensory stimuli. How such differences arise-at the level of sensory processing or in the circuits that generate behavior-remains largely unresolved across sensory modalities. We address this issue in the acoustic communication system of Drosophila. During courtship, males generate time-varying songs, and each sex responds with specific behaviors. We characterize male and female behavioral tuning for all aspects of song and show that feature tuning is similar between sexes, suggesting sex-shared song detectors drive divergent behaviors. We then identify higher-order neurons in the Drosophila brain, called pC2, that are tuned for multiple temporal aspects of one mode of the male's song and drive sex-specific behaviors. We thus uncover neurons that are specifically tuned to an acoustic communication signal and that reside at the sensory-motor interface, flexibly linking auditory perception with sex-specific behavioral responses.

Task-Dependent Changes in the Large-Scale Dynamics and Necessity of Cortical Regions.

Neural activity throughout the cortex is correlated with perceptual decisions, but inactivation studies suggest that only a small number of areas are necessary for these behaviors. Here we show that the number of required cortical areas and their dynamics vary across related tasks with different cognitive computations. In a visually guided virtual T-maze task, bilateral inactivation of only a few dorsal cortical regions impaired performance. In contrast, in tasks requiring evidence accumulation and/or post-stimulus memory, performance was impaired by inactivation of widespread cortical areas with diverse patterns of behavioral deficits across areas and tasks. Wide-field imaging revealed widespread ramps of Ca2+ activity during the accumulation and visually guided tasks. Additionally, during accumulation, different regions had more diverse activity profiles, leading to reduced inter-area correlations. Using a modular recurrent neural network model trained to perform analogous tasks, we argue that differences in computational strategies alone could explain these findings.

Specialized coding of sensory, motor and cognitive variables in VTA dopamine neurons.

There is increased appreciation that dopamine neurons in the midbrain respond not only to reward1 and reward-predicting cues1,2, but also to other variables such as the distance to reward3, movements4-9 and behavioural choices10,11. An important question is how the responses to these diverse variables are organized across the population of dopamine neurons. Whether individual dopamine neurons multiplex several variables, or whether there are subsets of neurons that are specialized in encoding specific behavioural variables remains unclear. This fundamental question has been difficult to resolve because recordings from large populations of individual dopamine neurons have not been performed in a behavioural task with sufficient complexity to examine these diverse variables simultaneously. Here, to address this gap, we used two-photon calcium imaging through an implanted lens to record the activity of more than 300 dopamine neurons from the ventral tegmental area of the mouse midbrain during a complex decision-making task. As mice navigated in a virtual-reality environment, dopamine neurons encoded an array of sensory, motor and cognitive variables. These responses were functionally clustered, such that subpopulations of neurons transmitted information about a subset of behavioural variables, in addition to encoding reward. These functional clusters were spatially organized, with neighbouring neurons more likely to be part of the same cluster. Together with the topography between dopamine neurons and their projections, this specialization and anatomical organization may aid downstream circuits in correctly interpreting the wide range of signals transmitted by dopamine neurons.

Imaging Cortical Dynamics in GCaMP Transgenic Rats with a Head-Mounted Widefield Macroscope.

Widefield imaging of calcium dynamics is an emerging method for mapping regional neural activity but is currently limited to restrained animals. Here we describe cScope, a head-mounted widefield macroscope developed to image large-scale cortical dynamics in rats during natural behavior. cScope provides a 7.8 × 4 mm field of view and dual illumination paths for both fluorescence and hemodynamic correction and can be fabricated at low cost using readily attainable components. We also report the development of Thy-1 transgenic rat strains with widespread neuronal expression of the calcium indicator GCaMP6f. We combined these two technologies to image large-scale calcium dynamics in the dorsal neocortex during a visual evidence accumulation task. Quantitative analysis of task-related dynamics revealed multiple regions having neural signals that encode behavioral choice and sensory evidence. Our results provide a new transgenic resource for calcium imaging in rats and extend the domain of head-mounted microscopes to larger-scale cortical dynamics. VIDEO ABSTRACT.

An Accumulation-of-Evidence Task Using Visual Pulses for Mice Navigating in Virtual Reality.

The gradual accumulation of sensory evidence is a crucial component of perceptual decision making, but its neural mechanisms are still poorly understood. Given the wide availability of genetic and optical tools for mice, they can be useful model organisms for the study of these phenomena; however, behavioral tools are largely lacking. Here, we describe a new evidence-accumulation task for head-fixed mice navigating in a virtual reality (VR) environment. As they navigate down the stem of a virtual T-maze, they see brief pulses of visual evidence on either side, and retrieve a reward on the arm with the highest number of pulses. The pulses occur randomly with Poisson statistics, yielding a diverse yet well-controlled stimulus set, making the data conducive to a variety of computational approaches. A large number of mice of different genotypes were able to learn and consistently perform the task, at levels similar to rats in analogous tasks. They are sensitive to side differences of a single pulse, and their memory of the cues is stable over time. Moreover, using non-parametric as well as modeling approaches, we show that the mice indeed accumulate evidence: they use multiple pulses of evidence from throughout the cue region of the maze to make their decision, albeit with a small overweighting of earlier cues, and their performance is affected by the magnitude but not the duration of evidence. Additionally, analysis of the mice's running patterns revealed that trajectories are fairly stereotyped yet modulated by the amount of sensory evidence, suggesting that the navigational component of this task may provide a continuous readout correlated to the underlying cognitive variables. Our task, which can be readily integrated with state-of-the-art techniques, is thus a valuable tool to study the circuit mechanisms and dynamics underlying perceptual decision making, particularly under more complex behavioral contexts.

Volumetric two-photon imaging of neurons using stereoscopy (vTwINS).

Two-photon laser scanning microscopy of calcium dynamics using fluorescent indicators is a widely used imaging method for large-scale recording of neural activity in vivo. Here, we introduce volumetric two-photon imaging of neurons using stereoscopy (vTwINS), a volumetric calcium imaging method that uses an elongated, V-shaped point spread function to image a 3D brain volume. Single neurons project to spatially displaced 'image pairs' in the resulting 2D image, and the separation distance between projections is proportional to depth in the volume. To demix the fluorescence time series of individual neurons, we introduce a modified orthogonal matching pursuit algorithm that also infers source locations within the 3D volume. We illustrated vTwINS by imaging neural population activity in the mouse primary visual cortex and hippocampus. Our results demonstrated that vTwINS provides an effective method for volumetric two-photon calcium imaging that increases the number of neurons recorded while maintaining a high frame rate.

Remodeling nuclear architecture allows efficient transport of herpesvirus capsids by diffusion.

The nuclear chromatin structure confines the movement of large macromolecular complexes to interchromatin corrals. Herpesvirus capsids of approximately 125 nm assemble in the nucleoplasm and must reach the nuclear membranes for egress. Previous studies concluded that nuclear herpesvirus capsid motility is active, directed, and based on nuclear filamentous actin, suggesting that large nuclear complexes need metabolic energy to escape nuclear entrapment. However, this hypothesis has recently been challenged. Commonly used microscopy techniques do not allow the imaging of rapid nuclear particle motility with sufficient spatiotemporal resolution. Here, we use a rotating, oblique light sheet, which we dubbed a ring-sheet, to image and track viral capsids with high temporal and spatial resolution. We do not find any evidence for directed transport. Instead, infection with different herpesviruses induced an enlargement of interchromatin domains and allowed particles to diffuse unrestricted over longer distances, thereby facilitating nuclear egress for a larger fraction of capsids.

Cellular mechanisms of alpha herpesvirus egress: live cell fluorescence microscopy of pseudorabies virus exocytosis.

Egress of newly assembled herpesvirus particles from infected cells is a highly dynamic process involving the host secretory pathway working in concert with viral components. To elucidate the location, dynamics, and molecular mechanisms of alpha herpesvirus egress, we developed a live-cell fluorescence microscopy method to visualize the final transport and exocytosis of pseudorabies virus (PRV) particles in non-polarized epithelial cells. This method is based on total internal reflection fluorescence (TIRF) microscopy to selectively image fluorescent virus particles near the plasma membrane, and takes advantage of a virus-encoded pH-sensitive probe to visualize the precise moment and location of particle exocytosis. We performed single-particle tracking and mean squared displacement analysis to characterize particle motion, and imaged a panel of cellular proteins to identify those spatially and dynamically associated with viral exocytosis. Based on our data, individual virus particles travel to the plasma membrane inside small, acidified secretory vesicles. Rab GTPases, Rab6a, Rab8a, and Rab11a, key regulators of the plasma membrane-directed secretory pathway, are present on the virus secretory vesicle. These vesicles undergo fast, directional transport directly to the site of exocytosis, which is most frequently near patches of LL5ß, part of a complex that anchors microtubules to the plasma membrane. Vesicles are tightly docked at the site of exocytosis for several seconds, and membrane fusion occurs, displacing the virion a small distance across the plasma membrane. After exocytosis, particles remain tightly confined on the outer cell surface. Based on recent reports in the cell biological and alpha herpesvirus literature, combined with our spatial and dynamic data on viral egress, we propose an integrated model that links together the intracellular transport pathways and exocytosis mechanisms that mediate alpha herpesvirus egress.

Nuclear herpesvirus capsid motility is not dependent on F-actin.

A considerable part of the herpesvirus life cycle takes place in the host nucleus. While much progress has been made to understand the molecular processes required for virus replication in the nucleus, much less is known about the temporal and spatial dynamics of these events. Previous studies have suggested that nuclear capsid motility is directed and dependent on actin filaments (F-actin), possibly using a myosin-based, ATP-dependent mechanism. However, the conclusions from these studies were indirect. They either relied on the effects of F-actin depolymerizing drugs to deduce an F-actin dependency or they visualized nuclear F-actin but failed to show a direct link to capsid motility. Moreover, no direct link between nuclear capsid motility and a molecular motor has been established. In this report, we reinvestigate the involvement of F-actin in nuclear herpesvirus capsid transport. We show for representative members of all three herpesvirus subfamilies that nuclear capsid motility is not dependent on nuclear F-actin and that herpesvirus infection does not induce nuclear F-actin in primary fibroblasts. Moreover, in these cells, three F-actin-inhibiting drugs failed to effect capsid motility. Only latrunculin A treatment stalled nuclear capsids but did so by an unexpected effect: the drug induced actin rods in the nucleus. Immobile capsids accumulated around actin rods, and immunoprecipitation experiments suggested that capsid motility stopped because latrunculin-induced actin rods nonspecifically bind nuclear capsids. Interestingly, capsid motility was unaffected in cells that do not induce actin rods. Based on these data, we conclude that herpesvirus nuclear capsid motility is not dependent on F-actin. Importance: Herpesviruses are large DNA viruses whose replication is dependent on the host nucleus. However, we do not understand how key nuclear processes, including capsid assembly, genome replication, capsid packaging, and nuclear egress, are dynamically connected in space and time. Fluorescence live-cell microscopy revealed that nuclear capsids are highly mobile early in infection. Two studies suggested that this motility might be due to active myosin-based transport of capsids on nuclear F-actin. However, direct evidence for such motor-based transport is lacking. We revisited this phenomenon and found no evidence that nuclear capsid motility depended on F-actin. Our results reopen the question of how nuclear herpesvirus capsids move in the host nucleus.

Germ plasm anchoring is a dynamic state that requires persistent trafficking.

Localized cytoplasmic determinants packaged as ribonucleoprotein (RNP) particles direct embryonic patterning and cell fate specification in a wide range of organisms. Once established, the asymmetric distributions of such RNP particles must be maintained, often over considerable developmental time. A striking example is the Drosophila germ plasm, which contains RNP particles whose localization to the posterior of the egg during oogenesis results in their asymmetric inheritance and segregation of germline from somatic fates in the embryo. Although actin-based anchoring mechanisms have been implicated, high-resolution live imaging revealed persistent trafficking of germ plasm RNP particles at the posterior cortex of the Drosophila oocyte. This motility relies on cortical microtubules, is mediated by kinesin and dynein motors, and requires coordination between the microtubule and actin cytoskeletons. Finally, we show that RNP particle motility is required for long-term germ plasm retention. We propose that anchoring is a dynamic state that renders asymmetries robust to developmental time and environmental perturbations.

In vivo imaging of alphaherpesvirus infection reveals synchronized activity dependent on axonal sorting of viral proteins.

A clinical hallmark of human alphaherpesvirus infections is peripheral pain or itching. Pseudorabies virus (PRV), a broad host range alphaherpesvirus, causes violent pruritus in many different animals, but the mechanism is unknown. Previous in vitro studies have shown that infected, cultured peripheral nervous system (PNS) neurons exhibited aberrant electrical activity after PRV infection due to the action of viral membrane fusion proteins, yet it is unclear if such activity occurs in infected PNS ganglia in living animals and if it correlates with disease symptoms. Using two-photon microscopy, we imaged autonomic ganglia in living mice infected with PRV strains expressing GCaMP3, a genetically encoded calcium indicator, and used the changes in calcium flux to monitor the activity of many neurons simultaneously with single-cell resolution. Infection with virulent PRV caused these PNS neurons to fire synchronously and cyclically in highly correlated patterns among infected neurons. This activity persisted even when we severed the presynaptic axons, showing that infection-induced firing is independent of input from presynaptic brainstem neurons. This activity was not observed after infections with an attenuated PRV recombinant used for circuit tracing or with PRV mutants lacking either viral glycoprotein B, required for membrane fusion, or viral membrane protein Us9, required for sorting virions and viral glycoproteins into axons. We propose that the viral fusion proteins produced by virulent PRV infection induce electrical coupling in unmyelinated axons in vivo. This action would then give rise to the synchronous and cyclical activity in the ganglia and contribute to the characteristic peripheral neuropathy.

Volume conservation principle involved in cell lengthening and nucleus movement during tissue morphogenesis.

Tissue morphogenesis is the process in which coordinated movements and shape changes of large numbers of cells form tissues, organs, and the internal body structure. Understanding morphogenetic movements requires precise measurements of whole-cell shape changes over time. Tissue folding and invagination are thought to be facilitated by apical constriction, but the mechanism by which changes near the apical cell surface affect changes along the entire apical-basal axis of the cell remains elusive. Here, we developed Embryo Development Geometry Explorer, an approach for quantifying rapid whole-cell shape changes over time, and we combined it with deep-tissue time-lapse imaging based on fast two-photon microscopy to study Drosophila ventral furrow formation. We found that both the cell lengthening along the apical-basal axis and the movement of the nucleus to the basal side proceeded stepwise and were correlated with apical constriction. Moreover, cell volume lost apically due to constriction largely balanced the volume gained basally by cell lengthening. The volume above the nucleus was conserved during its basal movement. Both apical volume loss and cell lengthening were absent in mutants showing deficits in the contractile cytoskeleton underlying apical constriction. We conclude that a single mechanical mechanism involving volume conservation and apical constriction-induced basal movement of cytoplasm accounts quantitatively for the cell shape changes and the nucleus movement in Drosophila ventral furrow formation. Our study provides a comprehensive quantitative analysis of the fast dynamics of whole-cell shape changes during tissue folding and points to a simplified model for Drosophila gastrulation.

Imaging cilia in zebrafish.

Research focused on cilia as extremely important cellular organelles has flourished in recent years. A thorough understanding of cilia regulation and function is critical, as disruptions of cilia structure and/or function have been linked to numerous human diseases and disorders. The tropical freshwater zebrafish is an excellent model organism in which to study cilia structure and function. We can readily image cilia and their motility in embryonic structures including Kupffer's vesicle during somite stages and the pronephros from 1 day postfertilization onward. Here, we describe how to image cilia by whole-mount immunofluorescence, transverse cryosection/immunohistochemistry, and transmission electron microscopy. We also describe how to obtain videos of cilia motility in living embryos.