Cell | Luo Liqun/Li Tongchao use in vivo imaging to reveal the developmental mechanism of the olfactory neural circuit in Drosophila

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During the brain development process of iNature, a large number of neurons are searching for the neurons that need to be connected in the brain at the same time.
How do they accurately locate the destination? On September 21, 2021, Luo Liqun and Li Tongchao from Stanford University jointly published a research paper titled "Cellular bases of olfactory circuit assembly revealed by systematic time-lapse imaging" in Cell, which uses the olfactory circuit of Drosophila to study dynamic cells Processes through which olfactory receptor neurons (ORN) precisely target axons to specific glomeruli in the ipsilateral and contralateral antennal lobe
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Time-lapse imaging of individual axons from 30 ORN types reveals the rich diversity of extension speed, innervation time, and ipsilateral branch location, and confirms that ipsilateral targeting occurs through stable transient qualitative branches
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Fast imaging using adaptive optics-corrected lattice-layer light microscopes showed that many ORN types that exhibit "exploratory branches" when approaching the target are composed of parallel microtubule end branches emanating from the F-actin-rich center
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Antennae nerve ablation reveals the important role of bilateral axons in contralateral target selection and ORN axons in promoting dendritic refinement of postsynaptic partner neurons
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In summary, these observations provide a cellular basis for the establishment of neuron wiring specificity
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The normal function of the nervous system depends on the precise assembly of neural circuits
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During development, a single neuron extends its axons and dendrites to match its synaptic partner
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The axon is guided by the growth cone, which navigates the complex extracellular environment at every step of its journey
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Many neurons perform this operation simultaneously in any given nerve area
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Although great progress has been made in identifying molecules that control axon guidance, dendritic formation, and target selection in the past few decades, the features that most of the wiring molecules play in the circuit assembly process have not been well characterized.

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Due to its high temporal resolution, time-lapse imaging has been used to define key cellular events in neuron wiring
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Notable examples include the discovery of growth cone dynamics in tissue culture; identification of axon-guided landmark cells in grasshopper limb buds; characterization of growth cone dynamics of retinal ganglion cells at the optic chiasm of mice; identification of coverage of fruit flies and zebras The repulsive interaction between sensory dendrites and axons on the body surface of fish and Caenorhabditis elegans; study the relationship between dendritic growth and synapse formation in the retinal protection system of fish and amphibians
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Most research has focused on a group of cells at a specific developmental stage
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It is not clear how the growth cone dynamics of the same neuron changes at different stages of circuit assembly, and the extent to which different types of neurons in the same circuit follow the same rules
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The article pattern (picture from Cell) The fruit fly olfactory system has become a model for studying the mechanism of neural circuit assembly
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The axons of 50 kinds of olfactory receptor neurons (ORN) and the dendrites of 50 kinds of projection neurons (PN) form a one-to-one connection in 50 discrete, stereotyped, and individually identifiable lobes in the antennal lobe.
Transmit olfactory information from pe-the maturation of the brain
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In the assembly process of the adult olfactory system, PN first extends dendrites to build a rough picture
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Then, ORN axons choose dorsolateral or ventromedial trajectories to surround the antennal lobe, cross the midline, and invade the ipsilateral and contralateral antennal lobe to find their synaptic partners
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A variety of cellular and molecular mechanisms have been identified to directly target selected dendrites and axons of the PN and ORN types; some of the mechanisms first discovered in the olfactory system of Drosophila were later discovered to be conserved when connected to the mammalian brain
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Here, based on the methods previously used to study the visual system of Drosophila in explant culture, the research developed an antenna brain explant preparation that summarizes the assembly of the olfactory circuit in the body through it
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The high-resolution adaptive optics lattice light sheet microscope (AO-LLSM) enables this study to discover the axon end structure before the ORN axon reaches the target
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The study also found that the cytoskeletal organization of ORN axon ends is very different from the cytoskeletal organization of the classic growth cone of neurons in primary culture
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Finally, ORN axon ablation revealed the important role of bilateral ORN axons in the selection of contralateral targets, and the important role of ORN axons in promoting dendritic refinement of PN
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In short, these observations provide a cellular basis for the establishment of neuron wiring specificity
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