Chi-Hon Lee

Eunice Kennedy Shriver National Institute of Child Health and Human Development, Maryland, United States

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Publications (22)188.8 Total impact

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    ABSTRACT: Many visual animals have innate preferences for particular wavelengths of light, which can be modified by learning. Drosophila's preference for UV over visible light requires UV-sensing R7 photoreceptors and specific wide-field amacrine neurons called Dm8. Here we identify three types of medulla projection neurons downstream of R7 and Dm8 and show that selectively inactivating one of them (Tm5c) abolishes UV preference. Using a modified GRASP method to probe synaptic connections at the single-cell level, we reveal that each Dm8 neuron forms multiple synaptic contacts with Tm5c in the center of Dm8's dendritic field but sparse connections in the periphery. By single-cell transcript profiling and RNAi-mediated knockdown, we determine that Tm5c uses the kainate receptor Clumsy to receive excitatory glutamate input from Dm8. We conclude that R7s→Dm8→Tm5c form a hard-wired glutamatergic circuit that mediates UV preference by pooling ∼16 R7 signals for transfer to the lobula, a higher visual center.
    Neuron 02/2014; 81(3):603-15. · 15.77 Impact Factor
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    ABSTRACT: How neurons form appropriately sized dendritic fields to encounter their presynaptic partners is poorly understood. The Drosophila medulla is organized in layers and columns and innervated by medulla neuron dendrites and photoreceptor axons. Here, we show that three types of medulla projection (Tm) neurons extend their dendrites in stereotyped directions and to distinct layers within a single column for processing retinotopic information. In contrast, the Dm8 amacrine neurons form a wide dendritic field to receive ∼16 R7 photoreceptor inputs. R7- and R8-derived Activin selectively restricts the dendritic fields of their respective postsynaptic partners, Dm8 and Tm20, to the size appropriate for their functions. Canonical Activin signaling promotes dendritic termination without affecting dendritic routing direction or layer. Tm20 neurons lacking Activin signaling expanded their dendritic fields and aberrantly synapsed with neighboring photoreceptors. We suggest that afferent-derived Activin regulates the dendritic field size of their postsynaptic partners to ensure appropriate synaptic partnership.
    Neuron 01/2014; · 15.77 Impact Factor
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    ABSTRACT: Analysis of cis-regulatory enhancers has revealed that they consist of clustered blocks of highly conserved sequences. Although most characterized enhancers reside near their target genes, a growing number of studies have shown that enhancers located over 50 kb from their minimal promoter(s) are required for appropriate gene expression and many of these 'long-range' enhancers are found in genomic regions that are devoid of identified exons. To gain insight into the complexity of Drosophila cis-regulatory sequences within exon-poor regions, we have undertaken an evolutionary analysis of 39 of these regions located throughout the genome. This survey revealed that within these genomic expanses, clusters of conserved sequence blocks (CSBs) are positioned once every 1.1 kb, on average, and that a typical cluster contains multiple (5 to 30 or more) CSBs that have been maintained for at least 190 My of evolutionary divergence. As an initial step toward assessing the cis-regulatory activity of conserved clusters within gene-free genomic expanses, we have tested the in-vivo enhancer activity of 19 consecutive CSB clusters located in the middle of a 115 kb gene-poor region on the 3(rd) chromosome. Our studies revealed that each cluster functions independently as a specific spatial/temporal enhancer. In total, the enhancers possess a diversity of regulatory functions, including dynamically activating expression in defined patterns within subsets of cells in discrete regions of the embryo, larvae and/or adult. We also observed that many of the enhancers are multifunctional-that is, they activate expression during multiple developmental stages. By extending these results to the rest of the Drosophila genome, which contains over 70,000 non-coding CSB clusters, we suggest that most function as enhancers.
    PLoS ONE 01/2013; 8(4):e60137. · 3.73 Impact Factor
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    ABSTRACT: Color and motion information are thought to be channeled through separate neural pathways, but it remains unclear whether and how these pathways interact to improve motion perception. In insects, such as Drosophila, it has long been believed that motion information is fed exclusively by one spectral class of photoreceptor, so-called R1 to R6 cells; whereas R7 and R8 photoreceptors, which exist in multiple spectral classes, subserve color vision. Here, we report that R7 and R8 also contribute to the motion pathway. By using electrophysiological, optical, and behavioral assays, we found that R7/R8 information converge with and shape the motion pathway output, explaining flies' broadly tuned optomotor behavior by its composite responses. Our results demonstrate that inputs from photoreceptors of different spectral sensitivities improve motion discrimination, increasing robustness of perception.
    Science 05/2012; 336(6083):925-31. · 31.20 Impact Factor
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    ABSTRACT: Phylogenetic footprinting has revealed that cis-regulatory enhancers consist of conserved DNA sequence clusters (CSCs). Currently, there is no systematic approach for enhancer discovery and analysis that takes full-advantage of the sequence information within enhancer CSCs. We have generated a Drosophila genome-wide database of conserved DNA consisting of >100,000 CSCs derived from EvoPrints spanning over 90% of the genome. cis-Decoder database search and alignment algorithms enable the discovery of functionally related enhancers. The program first identifies conserved repeat elements within an input enhancer and then searches the database for CSCs that score highly against the input CSC. Scoring is based on shared repeats as well as uniquely shared matches, and includes measures of the balance of shared elements, a diagnostic that has proven to be useful in predicting cis-regulatory function. To demonstrate the utility of these tools, a temporally-restricted CNS neuroblast enhancer was used to identify other functionally related enhancers and analyze their structural organization. cis-Decoder reveals that co-regulating enhancers consist of combinations of overlapping shared sequence elements, providing insights into the mode of integration of multiple regulating transcription factors. The database and accompanying algorithms should prove useful in the discovery and analysis of enhancers involved in any developmental process.
    Developmental Dynamics 01/2012; 241(1):169-89. · 2.59 Impact Factor
  • Ian A Meinertzhagen, Chi-Hon Lee
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    ABSTRACT: Fly and vertebrate nervous systems share many organizational features, such as layers, columns and glomeruli, and utilize similar synaptic components, such as ion channels and receptors. Both also exhibit similar network features. Recent technological advances, especially in electron microscopy, now allow us to determine synaptic circuits and identify pathways cell-by-cell, as part of the fly's connectome. Genetic tools provide the means to identify synaptic components, as well as to record and manipulate neuronal activity, adding function to the connectome. This review discusses technical advances in these emerging areas of functional connectomics, offering prognoses in each and identifying the challenges in bridging structural connectomics to molecular biology and synaptic physiology, thereby determining fundamental mechanisms of neural computation that underlie behavior.
    Advances in genetics 01/2012; 80:99-151. · 4.85 Impact Factor
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    ABSTRACT: Detecting motion is a feature of all advanced visual systems [1], nowhere more so than in flying animals, like insects [2, 3]. In flies, an influential autocorrelation model for motion detection, the elementary motion detector circuit (EMD; [4, 5]), compares visual signals from neighboring photoreceptors to derive information on motion direction and velocity. This information is fed by two types of interneuron, L1 and L2, in the first optic neuropile, or lamina, to downstream local motion detectors in columns of the second neuropile, the medulla. Despite receiving carefully matched photoreceptor inputs, L1 and L2 drive distinct, separable pathways responding preferentially to moving "on" and "off" edges, respectively [6, 7]. Our serial electron microscopy (EM) identifies two types of transmedulla (Tm) target neurons, Tm1 and Tm2, that receive apparently matched synaptic inputs from L2. Tm2 neurons also receive inputs from two retinotopically posterior neighboring columns via L4, a third type of lamina neuron. Light microscopy reveals that the connections in these L2/L4/Tm2 circuits are highly determinate. Single-cell transcript profiling suggests that nicotinic acetylcholine receptors mediate transmission within the L2/L4/Tm2 circuits, whereas L1 is apparently glutamatergic. We propose that Tm2 integrates sign-conserving inputs from neighboring columns to mediate the detection of front-to-back motion generated during forward motion.
    Current biology: CB 11/2011; 21(24):2077-84. · 10.99 Impact Factor
  • Krishna V Melnattur, Chi-Hon Lee
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    ABSTRACT: Both insect and vertebrate visual circuits are organized into orderly arrays of columnar and layered synaptic units that correspond to the array of photoreceptors in the eye. Recent genetic studies in Drosophila have yielded insights into the molecular and cellular mechanisms that pattern the layers and columns and establish specific connections within the synaptic units. A sequence of inductive events and complex cellular interactions coordinates the assembly of visual circuits. Photoreceptor-derived ligands, such as hedgehog and Jelly-Belly, induce target development and expression of specific adhesion molecules, which in turn serve as guidance cues for photoreceptor axons. Afferents are directed to specific layers by adhesive afferent-target interactions mediated by leucine-rich repeat proteins and cadherins, which are restricted spatially and/or modulated dynamically. Afferents are restricted to their topographically appropriate columns by repulsive interactions between afferents and by autocrine activin signaling. Finally, Dscam-mediated repulsive interactions between target neuron dendrites ensure appropriate combinations of postsynaptic elements at synapses. Essentially, all these Drosophila molecules have vertebrate homologs, some of which are known to carry out analogous functions. Thus, the studies of Drosophila visual circuit development would shed light on neural circuit assembly in general.
    Developmental Neurobiology 04/2011; 71(12):1286-96. · 4.42 Impact Factor
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    ABSTRACT: Here we report the development of a ternary version of the LexA::VP16/LexAop system in which the DNA-binding and trans-activating moieties are independently targeted using distinct promoters to achieve highly restricted, intersectional expression patterns. This Split LexA system can be concatenated with the Gal4/upstream activating sequence system to refine the expression patterns of existing Gal4 lines with minimal genetic manipulations.
    Genetics 03/2011; 188(1):229-33. · 4.39 Impact Factor
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    ABSTRACT: Metazoan development requires complex mechanisms to generate cells with diverse function. Alternative splicing of pre-mRNA not only expands proteomic diversity but also provides a means to regulate tissue-specific molecular expression. The N-Cadherin gene in Drosophila contains three pairs of mutually-exclusive alternatively-spliced exons (MEs). However, no significant differences among the resulting protein isoforms have been successfully demonstrated in vivo. Furthermore, while the N-Cadherin gene products exhibit a complex spatiotemporal expression pattern within embryos, its underlying mechanisms and significance remain unknown. Here, we present results that suggest a critical role for alternative splicing in producing a crucial and reproducible complexity in the expression pattern of arthropod N-Cadherin. We demonstrate that the arthropod N-Cadherin gene has maintained the three sets of MEs for over 400 million years using in silico and in vivo approaches. Expression of isoforms derived from these MEs receives precise spatiotemporal control critical during development. Both Drosophila and Tribolium use ME-13a and ME-13b in "neural" and "mesodermal" splice variants, respectively. As proteins, either ME-13a- or ME-13b-containing isoform can cell-autonomously rescue the embryonic lethality caused by genetic loss of N-Cadherin. Ectopic muscle expression of either isoform beyond the time it normally ceases leads to paralysis and lethality. Together, our results offer an example of well-conserved alternative splicing increasing cellular diversity in metazoans.
    PLoS Genetics 05/2009; 5(4):e1000441. · 8.52 Impact Factor
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    ABSTRACT: The shape of a neuron, its morphological signature, dictates the neuron's function by establishing its synaptic partnerships. Here, we review various anatomical methods used to reveal neuron shape and the contributions these have made to our current understanding of neural function in the Drosophila brain, especially the optic lobe. These methods, including Golgi impregnation, genetic reporters, and electron microscopy (EM), necessarily incorporate biases of various sorts that are easy to overlook, but that filter the morphological signatures we see. Nonetheless, the application of these methods to the optic lobe has led to reassuringly congruent findings on the number and shapes of neurons and their connection patterns, indicating that morphological classes are actually genetic classes. Genetic methods using, especially, GAL4 drivers and associated reporters have largely superceded classical Golgi methods for cellular analyses and, moreover, allow the manipulation of neuronal activity, thus enabling us to establish a bridge between morphological studies and functional ones. While serial-EM reconstruction remains the only reliable, albeit labor-intensive, method to determine actual synaptic connections, genetic approaches in combination with EM or high-resolution light microscopic techniques are promising methods for the rapid determination of synaptic circuit function.
    Journal of neurogenetics 02/2009; 23(1-2):68-77. · 0.73 Impact Factor
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    ABSTRACT: Drosophila vision is mediated by inputs from three types of photoreceptor neurons; R1-R6 mediate achromatic motion detection, while R7 and R8 constitute two chromatic channels. Neural circuits for processing chromatic information are not known. Here, we identified the first-order interneurons downstream of the chromatic channels. Serial EM revealed that small-field projection neurons Tm5 and Tm9 receive direct synaptic input from R7 and R8, respectively, and indirect input from R1-R6, qualifying them to function as color-opponent neurons. Wide-field Dm8 amacrine neurons receive input from 13-16 UV-sensing R7s and provide output to projection neurons. Using a combinatorial expression system to manipulate activity in different neuron subtypes, we determined that Dm8 neurons are necessary and sufficient for flies to exhibit phototaxis toward ultraviolet instead of green light. We propose that Dm8 sacrifices spatial resolution for sensitivity by relaying signals from multiple R7s to projection neurons, which then provide output to higher visual centers.
    Neuron 11/2008; 60(2):328-42. · 15.77 Impact Factor
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    ABSTRACT: Early sensory processing can play a critical role in sensing environmental cues. We have investigated the physiological and behavioral function of gain control at the first synapse of olfactory processing in Drosophila. Olfactory receptor neurons (ORNs) express the GABA(B) receptor (GABA(B)R), and its expression expands the dynamic range of ORN synaptic transmission that is preserved in projection neuron responses. Strikingly, each ORN channel has a unique baseline level of GABA(B)R expression. ORNs that sense the aversive odorant CO(2) do not express GABA(B)Rs and do not have significant presynaptic inhibition. In contrast, pheromone-sensing ORNs express a high level of GABA(B)Rs and exhibit strong presynaptic inhibition. Furthermore, pheromone-dependent mate localization is impaired in flies that lack GABA(B)Rs in specific ORNs. These findings indicate that different olfactory receptor channels employ heterogeneous presynaptic gain control as a mechanism to allow an animal's innate behavioral responses to match its ecological needs.
    Neuron 08/2008; 59(2):311-21. · 15.77 Impact Factor
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    ABSTRACT: The organization of neuronal wiring into layers and columns is a common feature of both vertebrate and invertebrate brains. In the Drosophila visual system, each R7 photoreceptor axon projects within a single column to a specific layer of the optic lobe. We refer to the restriction of terminals to single columns as tiling. In a genetic screen based on an R7-dependent behavior, we identified the Activin receptor Baboon and the nuclear import adaptor Importin-alpha3 as being required to prevent R7 axon terminals from overlapping with the terminals of R7s in neighboring columns. This tiling function requires the Baboon ligand, dActivin, the transcription factor, dSmad2, and retrograde transport from the growth cone to the R7 nucleus. We propose that dActivin is an autocrine signal that restricts R7 growth cone motility, and we demonstrate that it acts in parallel with a paracrine signal that mediates repulsion between R7 terminals.
    Neuron 01/2008; 56(5):793-806. · 15.77 Impact Factor
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    ABSTRACT: In the eye, visual information is segregated into modalities such as color and motion, these being transferred to the central brain through separate channels. Here, we genetically dissect the achromatic motion channel in the fly Drosophila melanogaster at the level of the first relay station in the brain, the lamina, where it is split into four parallel pathways (L1-L3, amc/T1). The functional relevance of this divergence is little understood. We now show that the two most prominent pathways, L1 and L2, together are necessary and largely sufficient for motion-dependent behavior. At high pattern contrast, the two pathways are redundant. At intermediate contrast, they mediate motion stimuli of opposite polarity, L2 front-to-back, L1 back-to-front motion. At low contrast, L1 and L2 depend upon each other for motion processing. Of the two minor pathways, amc/T1 specifically enhances the L1 pathway at intermediate contrast. L3 appears not to contribute to motion but to orientation behavior.
    Neuron 11/2007; 56(1):155-70. · 15.77 Impact Factor
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    ABSTRACT: Drosophila N-cadherin (CadN) is an evolutionarily conserved, atypical classical cadherin, which has a large complex extracellular domain and a catenin-binding cytoplasmic domain. We have previously shown that CadN regulates target selection of R7 photoreceptor axons. To determine the functional domains of CadN, we conducted a structure-function analysis focusing on its in vitro adhesive activity and in vivo function in R7 growth cones. We found that the cytoplasmic domain of CadN is largely dispensable for the targeting of R7 growth cones, and it is not essential for mediating homophilic interaction in cultured cells. Instead, the cytoplasmic domain of CadN is required for maintaining proper growth cone morphology. Domain swapping with the extracellular domain of CadN2, a related but non-adhesive cadherin, revealed that the CadN extracellular domain is required for both adhesive activity and R7 targeting. Using a target-mosaic system, we generated CadN mutant clones in the optic lobe and examined the target-selection of genetically wild-type R7 growth cones to CadN mutant target neurons. We found that CadN, but neither LAR nor Liprin-alpha, is required in the medulla neurons for R7 growth cones to select the correct medulla layer. Together, these data suggest that CadN mediates homophilic adhesive interactions between R7 growth cones and medulla neurons to regulate layer-specific target selection.
    Developmental Biology 05/2007; 304(2):759-70. · 3.87 Impact Factor
  • Chun-Yuan Ting, Chi-Hon Lee
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    ABSTRACT: Fly visual circuits are organized into lattice-like arrays and layers. Recent genetic studies have provided insights into how these reiterated structures are assembled through stepwise processes and how precise connections are established during development. Afferent-derived morphogens, such as Hedgehog, play a key role in organizing the overall structure by inducing and recruiting target neurons and glia. In turn, the target-derived ligand DWnt4 guides Frizzled2-expressing photoreceptor afferents to their proper destination. Photoreceptor afferents select specific synaptic targets by forming adhesive interactions and regulating actin cytoskeleton in growth cones. Target specificity is probably achieved by restricting the expression of adhesive molecules, such as Capricious, to appropriate presynaptic and postsynaptic partners, and by differentially regulating the function of broadly expressed adhesive molecules such as N-cadherin.
    Current Opinion in Neurobiology 03/2007; 17(1):65-72. · 7.34 Impact Factor
  • Developmental Biology - DEVELOP BIOL. 01/2007; 306(1):376-376.

Publication Stats

444 Citations
319 Downloads
1k Views
188.80 Total Impact Points

Institutions

  • 2011–2014
    • Eunice Kennedy Shriver National Institute of Child Health and Human Development
      Maryland, United States
  • 2005–2013
    • National Institute of Child Health and Human Development
      Maryland, United States
  • 2009–2012
    • Dalhousie University
      Halifax, Nova Scotia, Canada