The diversity of lobula plate tangential cells (LPTCs) in the Drosophila motion vision system

Abstract

To navigate through the environment, animals rely on visual feedback to control their movements relative to their surroundings. In dipteran flies, visual feedback is provided by the wide-field motion-sensitive neurons in the visual system called lobula plate tangential cells (LPTCs). Understanding the role of LPTCs in fly behaviors can address many fundamental questions on how sensory circuits guide behaviors. The blowfly was estimated to have ~ 60 LPTCs, but only a few have been identified in Drosophila. We conducted a Gal4 driver screen and identified five LPTC subtypes in Drosophila, based on their morphological characteristics: LPTCs have large arborizations in the lobula plate and project to the central brain. We compared their morphologies to the blowfly LPTCs and named them after the most similar blowfly cells: CH, H1, H2, FD1 and FD3, and V1. We further characterized their pre- and post-synaptic organizations, as well as their neurotransmitter profiles. These anatomical features largely agree with the anatomy and function of their likely blowfly counterparts. Nevertheless, several anatomical details indicate the Drosophila LPTCs may have more complex functions. Our characterization of these five LPTCs in Drosophila will facilitate further functional studies to understand their roles in the visual circuits that instruct fly behaviors.

Keywords: Drosophila; LPTC; Lobula plate; Motion vision; Optomotor.

Figure 1.. Characterization of CH-like neurons.

a. A pair of CH-like neurons on each side of the brain. Arrowheads point out the cell bodies. They have processes in both the LOP and IPS. The driver R35A10-Gal4 was crossed to MCFO3 to only show the CH-like neurons. (Blue: anti-nc82 staining.) b. Single-cell morphology of a dCH-like neuron (up) and a vCH-like neuron (down). These silhouettes are obtained by cropping single-channel images of MCFO staining with Fiji ImageJ (up: MCFO1; down MCFO3). c. Drawing of the blowfly CH neurons. Modified from Fig.1 in (Eckert and Dvorak 1983). d. CH-like neurons reside in layer 1 of the LOP (upper panel: staining of LOP layers obtained by imaging the brain along the DV-axis; lower panel: schematic of LOP layers). e. CH-like neurons have postsynaptic processes (labeled by DMK) in the IPS and a mixture of pre- and post-synaptic processes (labeled by both SyteGFP and DMK) in the LOP. f. Intersection of R35A10-Gal4 with GAD1. Arrowheads point to the cell bodies of CH-like neurons. The intersection also captures some small medulla neurons. MCFO= MultiColor FlpOut; LOP = lobula plate; IPS = inferior posterior slope; DMK = DenMark; SyteGFP = Synaptotagmin eGFP.

Figure 2.. Characterization of H1-like neuron.

a. Staining of VT045663-Gal4 driving 20xUAS-6XGFP. Arrowheads point to the cell bodies of H1-like neurons on each side. The ipsilateral H1-like neuron has processes that overlay the processes from the contralateral H1-like neuron. b. Single-cell morphology of an H1-neuron. The image was obtained by cropping out the H1-like neuron from a VT045663-Gal4 > 20X UAS-6XGFP staining that happened to only label the H1-like neuron on one side of the brain. c. Sketch of a single blowfly H1 neuron. Modified from Fig.2 in (Eckert 1980). d. The ipsilateral processes (labeled by DMK) of H1-like neuron reside in layer 2 of the LOP. Pre-synaptic contralateral processes (labeled by SyteGFP) are in both layer 1 and 2 of the LOP. e. Ipsilateral LOP processes express DMK and are postsynaptic while contralateral processes in LOP express presynaptic SyteGFP. f. The expression of H1-like neuronal driver colocalizes with a dvGlut driver. i.p = ipsilateral; c.l. = contralateral; LOP = lobula plate; DMK = DenMark; SyteGFP = Synaptotagmin eGFP; dvGlut = the Drosophila vesicular glutamate transporter.

Figure 3.. Characterization of an H2-like neuron.

a. Staining of R47F01-Gal4 driving 20XUAS-6XGFP. Arrowheads point to the cell bodies of H2-like neurons on each side of the brain. Its central brain branch terminates in IPS. b. Single-cell morphology of an H2-like neuron. The silhouette was obtained by cropping out the neuron from the staining of R47F01-Gal4 driving UAS-CD8-RFP with Fiji ImageJ. c. Drawing of the blowfly H2 modified from Fig.1 in (Farrow et al. 2006). d. The H2-like neuron projects to layer 2 of the LOP. e. The LOP processes of the H2-like neuron express a mixture of postsynaptic DMK and presynaptic SyteGFP, whereas the central brain processes only express SyteGFP. f. Expression of the H2-like neuronal driver colocalizes with the expression of ChAT. LOP = lobula plate; IPS = inferior posterior slope; DMK = DenMark; SyteGFP = Synaptotagmin eGFP; ChAT: choline acetyltransferase.

Figure 4.. Characterization of FD1-like and FD3-like neurons

a. Staining of R14C03-Gal4 crossed to MCFO4. Two FD1-like neurons are labeled on both sides of the brain. An FD3-like neuron is labeled on the left side. In the central brain, both neurons have ipsilateral branches in the PLP and contralateral projections to the IPS. b. Single-cell morphology of an FD1-like neuron (upper panel) and an FD3-like neuron (lower panel). These images are generated by cropping the neurons from sparse labeling staining obtained through crossing VT045663-Gal4 with FLEXAMP (a flip-out tool, see Methods; upper panel) or MCFO5 (lower panel). c. Drawing of the blowfly FD1 and FD3 modified from Fig6a and Fig13, respectively in (Egelhaaf 1985) d. FD1-like neuron occupies mainly layer 1 and 2 of the LOP, and only sparsely innervates layer 3. FD3-like neuron occupies layer 2 only. e. Both FD1-like and FD3-like cells express postsynaptic DMK in the LOP and in the ipsilateral IPS. Their projections to the contralateral central brain express presynaptic SyteGFP. f. Expression of FD1-like and FD3-like neurons colocalizes with the expression of ChAT. MCFO= MultiColor FlpOut; LOP = lobula plate; IPS = inferior posterior slope; PLP = posterior lateral protocerebrum; DMK = DenMark; SyteGFP = Synaptotagmin eGFP; ChAT: choline acetyltransferase.

Figure 5.. Characterization of a V1-like neuron

a. Single-cell morphology of a V1-like neuron. The image was obtained by crossing VT000771-Gal4 to MCFO4. The neuron has two branches in different parts of LOP, and its central brain processes are in PS. b. Sketch of the blowfly V1 modified from Fig.1a in (Haag and Borst 2008). Copyright 2008 Society for Neuroscience. c. The dorsal LOP branch of the V1-like neuron is in layer 3 of the LOP, whereas the ventral LOP branch is in layer 1. d. The ipsilateral processes in the central brain express postsynaptic DMK while the projection to the contralateral LOP expresses presynaptic SyteGFP. e. Expression of V1-like neuronal driver colocalizes with the expression of ChAT. MCFO= MultiColor FlpOut; LOP = lobula plate; PS = posterior slope; DMK = DenMark; SyteGFP = Synaptotagmin eGFP; ChAT: choline acetyltransferase.