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1AbstractBehaviour involving two or more individuals within the same species is known as social behaviour. Sexual dimorphisms and environmental cues as well as age, experience and social hierarchy shape social behaviour. The study of this complex behaviour, however, has one essential boundary condition: the distance between individuals. Because social signals (e.g. acoustic, visual or olfactory) have limited reach the inter-individual distance is crucial for the correct transmission of such signals. In this study we present a newly developed assay to study group behaviour and inter-individual distance in Drosophila melanogaster. This assay consists of a circular observation space flanked by two automatic release devices that allow flies to enter individually. By releasing the flies one at a time, the observer can study different features of (intra-)group behaviour with different group compositions. Importantly, the observer can control (manually or through automatisation) the increase of group size over time.Over the last decades insects became more relevant as convenient model organisms to study the neurophysiological and genetic basis of (human) neuro-developmental disorders. Drosophila models are used to study the genetic and neuronal underpinnings of a wide range of neurological disorders. In some cases the studies revealed alterations in social behaviour consistent with descriptions of behavioural symptoms in human disorders.Social behaviours in Drosophila are well-studied and include courtship, mating, aggression and group interactions. This setup will facilitate the analysis of these aspects of group interactions in Drosophila, allowing for a deeper understanding of the neuronal circuits and genetic factors involved in those behaviours.2Contribution to the fieldSocial behaviour pertains to the most sophisticated behavioural feats, as it involves multiple, interacting individuals. These complex interactions often conceal the underlying neuronal and ethological mechanisms. One of the most basal ethological mechanisms is the inter-individual distance, which resembles a perimeter in which each individual needs to formulate a response to the approach of others.We introduce a device that allows to test the inter-individual distance under consistent circumstances, by automating the entry time and direction of conspecifics into the arena. We can further observe the composition, dynamics and forming of larger animal groups as well as their separating. We can also observe how the individual distances alter during the process. Also other behaviours can be easily observed, e.g. aggression, courtship, homosexual courtship, etc. We successfully employed this approach in [1] and could discriminate the role of different neuroligins in social behaviour. We provide a detailed description including building plans and material lists for this social observation device. The system can be run in an automatic mode to ensure the consistency of experiments or in a manual mode to test animals under more flexible social situations. We provide multiple back lighting systems to test animals in the dark (infra-red LEDs) or in illuminated environments (vis. range LEDs). The system is fully automated and can be linked with a number of animal trackers (e.g. T-Rex, deeplabcut, LACE, etc.) via simple videography. We hope that our experimental setup augments the variety of behaviours testable in ethological setups (T-maze, water-mazes, operant conditing setups, etc) with social interaction and group formation.

This review aims to highlight the importance of saccades during locomotion as a strategy to reduce sensory information loss while the subject is moving. Acquiring sensory data from the environment during movement results in a temporal flow of information, as the sensory precept changes with the position of the observer. Accordingly, the movement pattern shapes the sensory flow. Therefore, the requirements of locomotion and sensation have to be balanced in the behaviour of the organism. Insect vision provides deep insight into the interplay between action and perception. Insects can shape their optic flow by reducing their rotational movements to fast and short saccades. This generates prolonged phases of translations which provide depth information. Extensive behavioural and physiological studies on insects show how shaping the optic flow facilitates the coding of motion vision. Indeed the saccadic strategy provides an elegant solution to optimise sensory flow. Complementary studies in other taxa reported similar locomotion strategies emphasising the crucial influence of sensory flow on locomotion.

Cholesterol is an essential membrane component enriched in plasma membranes, growth cones, and synapses. The brain normally synthesizes all cholesterol locally, but the contribution of individual cell types to brain cholesterol metabolism is unknown. To investigate whether cortical projection neuronsin vivoessentially require cholesterol biosynthesis and which cell types support neurons, we have conditionally ablated the cholesterol biosynthesis in these neurons in mice either embryonically or postnatally. We found that cortical projection neurons synthesize cholesterol during their entire lifetime. At all stages, they can also benefit from glial support. Adult neurons that lack cholesterol biosynthesis are mainly supported by astrocytes such that their functional integrity is preserved. In contrast, microglial cells support young neurons. However, compensatory efforts of microglia are only transient leading to layer-specific neuronal death and the reduction of cortical projections. Hence, during the phase of maximal membrane growth and maximal cholesterol demand, neuronal cholesterol biosynthesis is indispensable. Analysis of primary neurons revealed that neurons tolerate only slight alteration in the cholesterol content and plasma membrane tension. This quality control allows neurons to differentiate normally and adjusts the extent of neurite outgrowth, the number of functional growth cones and synapses to the available cholesterol. This study highlights both the flexibility and the limits of horizontal cholesterol transferin vivoand may have implications for the understanding of neurodegenerative diseases.

Duplication of PLP1 (proteolipid protein gene 1) and the subsequent overexpression of the myelin protein PLP (also known as DM20) in oligodendrocytes is the most frequent cause of Pelizaeus-Merzbacher disease (PMD), a fatal leukodystrophy without therapeutic options. PLP binds cholesterol and is contained within membrane lipid raft microdomains. Cholesterol availability is the rate-limiting factor of central nervous system myelin synthesis. Transgenic mice with extra copies of the Plp1 gene are accurate models of PMD. Dysmyelination followed by demyelination, secondary inflammation and axon damage contribute to the severe motor impairment in these mice. The finding that in Plp1-transgenic oligodendrocytes, PLP and cholesterol accumulate in late endosomes and lysosomes (endo/lysosomes), prompted us to further investigate the role of cholesterol in PMD. Here we show that cholesterol itself promotes normal PLP trafficking and that dietary cholesterol influences PMD pathology. In a preclinical trial, PMD mice were fed a cholesterol-enriched diet. This restored oligodendrocyte numbers and ameliorated intracellular PLP accumulation. Moreover, myelin content increased, inflammation and gliosis were reduced and motor defects improved. Even after onset of clinical symptoms, cholesterol treatment prevented disease progression. Dietary cholesterol did not reduce Plp1 overexpression but facilitated incorporation of PLP into myelin membranes. These findings may have implications for therapeutic interventions in patients with PMD.