Arrows indicate the dendrite’s preferred direction (PD) of motion. Yellow circles indicate the dendrite’s first branching point. Numbers indicate the four layers of the lobula plate. Drawing depicting the dendrite orientation and axon projection pattern defining each of the four T4/T5 neuron subtypes (a,b,c,d). We are currently attempting to understand the cellular rules and underlying molecular mechanisms by which the four T4/T5 neuron subtypes acquire their distinctly oriented dendritic arbors.įigure 3. Our working hypothesis is that the direction in which a T4/T5 dendrite grows during development determines the arrangement of its synaptic inputs, and thus its direction selectivity. Therefore, dendrite orientation appears strictly linked to directional tuning. How do these differences emerge during development? Interestingly, neurons of each T4/T5 subtype orient their dendrites in a subtype-specific manner (Takemura et al., 2017 Shinomiya et al., 2019). Differences between T4/T5 neuron subtypes in directional tuning are thought to result from differences in the spatial organization of synapses they receive from an identical set of input neurons (Serbe et al., 2016 Arenz et al., 2017 Takemura et al., 2017 Shinomiya et al., 2019). Furthermore, each T4/T5 subtype responds exclusively to motion in one of the four cardinal directions (Maisak et al., 2013). These defects together affect the optomotor response of adult flies.īoth T4 and T5 neurons can be further subdivided into four subtypes (a,b,c,d), each innervating only one of the four lobula plate layers (Figure 1) (Fischbach and Dittrich, 1989). This, in turn, induces a defect in the dendrites of lobula plate tangential cells (LPTCs), which are postsynaptic to T4/T5 neurons. Layer-specific innervation of T4/T5 dendrites and axons is lost upon SoxN or Sox102F knockdown. Drawing summarizing the anatomical and behavioral phenotypes observed upon specific disruption of SoxN or Sox102F in T4/T5 neurons. ![]() ![]() When this process fails, some postsynaptic partners of T4/T5 neurons also exhibit aberrant morphologies, which affects the proper functioning of the fly motion sensing circuits (Figure 2) (Schilling et al., 2019).įigure 2. We have found that SoxN and Sox102F, two members of the SOX family of transcription factors, are required in all T4 and T5 neurons during their maturation in order to restrict their dendrites and axons into single synaptic layers. A common attribute of T4 and T5 neurons is the layer-specific innervation displayed by their dendrites and axons (Figure 1) (Fischbach and Dittrich, 1989), which is thought to support precise synaptic connectivity. This specialization results from their dendrites arborizing in different neuropil regions (T4 dendrites in the medulla, T5 dendrites in the lobula) and therefore receiving synaptic input from different neuronal types. While T4 neurons respond to motion of brightness increments, T5 neurons detect motion of brightness decrements (Maisak et al., 2013). The dendrites of T4 and T5 neurons represent the first processing stage in the fly visual system in which the direction of image motion is encoded. To address this question, we use the T4/T5 neurons of the Drosophila visual system as a model because of their genetic accessibility and the extensive knowledge we have about their morphology and physiology. ![]() How neurons acquire a diverse repertoire of structural and functional properties in order to establish working neural circuits remains poorly understood. The axon terminal of the labelled T4 neuron is restricted to lobula plate layer 3. The four layers of the lobula plate are visualized by staining Bruchpilot (in magenta). Right panel: Drosophila optic lobe with a single T4 neuron labelled with GFP (in green). T4 dendrites arborize only in medulla layer M10, while T5 dendrites arborize only in lobula layer Lo1. Connectin signal is very high in medulla layer M9 and lobula layer Lo3. ![]() Neuropil layers are visualized by staining DN-cadherin (in grey) and Connectin (in red). Left panel: Drosophila optic lobe with all T4/T5 neurons labelled with GFP (in green).
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