Most animals have compound eyes, with tens to thousands of lenses attached rigidly to the exoskeleton. A natural assumption is that all of these species must resort to moving either their head or their body to actively change their visual input. However, classic anatomy has revealed that flies have muscles poised to move their retinas under the stable lenses of each compound eye. Here we show that Drosophila use their retinal muscles to smoothly track visual motion, which helps to stabilize the retinal image, and also to perform small saccades when viewing a stationary scene. We show that when the retina moves, visual receptive fields shift accordingly, and that even the smallest retinal saccades activate visual neurons. Using a head-fixed behavioural paradigm, we find that Drosophila perform binocular, vergence movements of their retinas-which could enhance depth perception-when crossing gaps, and impairing the physiology of retinal motor neurons alters gap-crossing trajectories during free behaviour. That flies evolved an ability to actuate their retinas suggests that moving the eye independently of the head is broadly paramount for animals. The similarities of smooth and saccadic movements of the Drosophila retina and the vertebrate eye highlight a notable example of convergent evolution.

Muscle strength testing is routine in clinical practice. Here we provide an aid to the documentation and visual conceptualization of those results - MuscleViz: a free, open-source application for visualizing the results of muscle strength testing. Its use in clinical settings streamlines the communication of physical examination findings. The tool is also useful for presenting patient data in case reports or case series. A push towards free, open-source software has benefitted other areas of science; we believe a similar effort dedicated to the development of clinical tools is worth pursuing.

Aversive olfactory memory is formed in the mushroom bodies in Drosophila melanogaster. Memory retrieval requires mushroom body output, but the manner in which a memory trace in the mushroom body drives conditioned avoidance of a learned odor remains unknown. To identify neurons that are involved in olfactory memory retrieval, we performed an anatomical and functional screen of defined sets of mushroom body output neurons. We found that MB-V2 neurons were essential for retrieval of both short- and long-lasting memory, but not for memory formation or memory consolidation. MB-V2 neurons are cholinergic efferent neurons that project from the mushroom body vertical lobes to the middle superiormedial protocerebrum and the lateral horn. Notably, the odor response of MB-V2 neurons was modified after conditioning. As the lateral horn has been implicated in innate responses to repellent odorants, we propose that MB-V2 neurons recruit the olfactory pathway involved in innate odor avoidance during memory retrieval.

Animals discriminate stimuli, learn their predictive value and use this knowledge to modify their behavior. In Drosophila, the mushroom body (MB) plays a key role in these processes. Sensory stimuli are sparsely represented by ∼2000 Kenyon cells, which converge onto 34 output neurons (MBONs) of 21 types. We studied the role of MBONs in several associative learning tasks and in sleep regulation, revealing the extent to which information flow is segregated into distinct channels and suggesting possible roles for the multi-layered MBON network. We also show that optogenetic activation of MBONs can, depending on cell type, induce repulsion or attraction in flies. The behavioral effects of MBON perturbation are combinatorial, suggesting that the MBON ensemble collectively represents valence. We propose that local, stimulus-specific dopaminergic modulation selectively alters the balance within the MBON network for those stimuli. Our results suggest that valence encoded by the MBON ensemble biases memory-based action selection.

The role of gamma amino butyric acid (GABA) release and inhibitory neurotransmission in regulating most behaviors remains unclear. The vesicular GABA transporter (VGAT) is required for the storage of GABA in synaptic vesicles and provides a potentially useful probe for inhibitory circuits. However, specific pharmacologic agents for VGAT are not available, and VGAT knockout mice are embryonically lethal, thus precluding behavioral studies. We have identified the Drosophila ortholog of the vesicular GABA transporter gene (which we refer to as dVGAT), immunocytologically mapped dVGAT protein expression in the larva and adult and characterized a dVGAT(minos) mutant allele. dVGAT is embryonically lethal and we do not detect residual dVGAT expression, suggesting that it is either a strong hypomorph or a null. To investigate the function of VGAT and GABA signaling in adult visual flight behavior, we have selectively rescued the dVGAT mutant during development. We show that reduced GABA release does not compromise the active optomotor control of wide-field pattern motion. Conversely, reduced dVGAT expression disrupts normal object tracking and figure-ground discrimination. These results demonstrate that visual behaviors are segregated by the level of GABA signaling in flies, and more generally establish dVGAT as a model to study the contribution of GABA release to other complex behaviors.

A series of classical studies in non-human primates has revealed the neuronal activity patterns underlying decision-making. However, the circuit mechanisms for such patterns remain largely unknown. Recent detailed circuit analyses in simpler neural systems have started to reveal the connectivity patterns underlying analogous processes. Here we review a few of these systems that share a particular connectivity pattern, namely mutual inhibition of lateral inhibition. Close examination of these systems suggests that this recurring connectivity pattern ('network motif') is a building block to enforce particular dynamics, which can be used not only for simple behavioral choice but also for more complex choices and other brain functions. Thus, a network motif provides an elementary computation that is not specific to a particular brain function and serves as an elementary building block in the brain.

Cell reprogramming is thought to be associated with a full metabolic switch from an oxidative- to a glycolytic-based metabolism. However, neither the dynamics nor the factors controlling this metabolic switch are fully understood. By using cellular, biochemical, protein array, metabolomic, and respirometry analyses, we found that c-MYC establishes a robust bivalent energetics program early in cell reprogramming. Cells prone to undergo reprogramming exhibit high mitochondrial membrane potential and display a hybrid metabolism. We conclude that MYC proteins orchestrate a rewiring of somatic cell metabolism early in cell reprogramming, whereby somatic cells acquire the phenotypic plasticity necessary for their transition to pluripotency in response to either intrinsic or external cues.

50 years ago, Vincent Allfrey and colleagues discovered that lymphocyte activation triggers massive acetylation of chromatin. However, the molecular mechanisms driving epigenetic accessibility are still unknown. We here show that stimulated lymphocytes decondense chromatin by three differentially regulated steps. First, chromatin is repositioned away from the nuclear periphery in response to global acetylation. Second, histone nanodomain clusters decompact into mononucleosome fibers through a mechanism that requires Myc and continual energy input. Single-molecule imaging shows that this step lowers transcription factor residence time and non-specific collisions during sampling for DNA targets. Third, chromatin interactions shift from long range to predominantly short range, and CTCF-mediated loops and contact domains double in numbers. This architectural change facilitates cognate promoter-enhancer contacts and also requires Myc and continual ATP production. Our results thus define the nature and transcriptional impact of chromatin decondensation and reveal an unexpected role for Myc in the establishment of nuclear topology in mammalian cells.

The formation of the primitive heart tube from cardiomyocytes and endocardial cells is a key event in mammalian development. Previous studies suggested that cardiomyocytes and endocardial cells segregate from a shared cardiac progenitor around the onset of gastrulation, yet their lineage relationship with other mesodermal tissues remains unclear. Using retrospective and prospective clonal analyses in mouse embryos, we traced cardiomyocyte and endocardial progenitors from the primitive streak to the heart tube. Our results identify two independent mesodermal populations specified around gastrulation onset. While each of these populations is unipotent in producing cardiomyocytes or endocardium, they retain multipotency and contribute to different subsets of non-cardiac mesoderm. Nonetheless, live imaging identifies simultaneous ingression and intermingling of these two mesodermal lineages in the primitive streak, showing their coordinated specification and migration. The proposed model for cardiac progenitor specification will help understanding the origins of congenital heart diseases and designing tissue engineering strategies.

Class-18 myosins are most closely related to conventional class-2 nonmuscle myosins (NM2). Surprisingly, the purified head domains of Drosophila, mouse, and human myosin 18A (M18A) lack actin-activated ATPase activity and the ability to translocate actin filaments, suggesting that the functions of M18A in vivo do not depend on intrinsic motor activity. M18A has the longest coiled coil of any myosin outside of the class-2 myosins, suggesting that it might form bipolar filaments similar to conventional myosins. To address this possibility, we expressed and purified full-length mouse M18A using the baculovirus/Sf9 system. M18A did not form large bipolar filaments under any of the conditions tested. Instead, M18A formed an ∼65-nm-long bipolar structure with two heads at each end. Importantly, when NM2 was polymerized in the presence of M18A, the two myosins formed mixed bipolar filaments, as evidenced by cosedimentation, electron microscopy, and single-molecule imaging. Moreover, super-resolution imaging of NM2 and M18A using fluorescently tagged proteins and immunostaining of endogenous proteins showed that NM2 and M18A are present together within individual filaments inside living cells. Together, our in vitro and live-cell imaging data argue strongly that M18A coassembles with NM2 into mixed bipolar filaments. M18A could regulate the biophysical properties of these filaments and, by virtue of its extra N- and C-terminal domains, determine the localization and/or molecular interactions of the filaments. Given the numerous, fundamental cellular and developmental roles attributed to NM2, our results have far-reaching biological implications.