Assembly of the fly olfactory circuit

A central question in neural circuit assembly is how neurons connect specifically with their synaptic partners. We are using the fly olfactory circuit to investigate the general principles by which wiring specificity is established during development. The assembly of the fly olfactory circuit requires precise matching between axons from 50 olfactory receptor neuron types and dendrites from 50 projection neuron types (Fig. 1-1). In the past 20 years, we have identified key cellular interactions and molecular mechanisms at specific steps of olfactory circuit assembly (Fig. 1-2). More recently, we have also taken transcriptomic (Fig. 1-3), proteomic (Fig. 1-4), and live cell imaging (Movie 1-1) approaches to complement genetic analyses of individual genes. We are currently integrating these approaches to deepen our understanding of the combinatorial cell-surface codes that instruct connection specificity. For example, one goal is to completely switch connection specificity of one neuron type to that of another.

Our studies have made the fly olfactory circuit one of the best characterized systems in terms of cellular and molecular mechanisms of circuit assembly. These investigations have also identified evolutionarily conserved wiring molecules that play similar functions in the mouse brain (see Section 2).

Selected publications

Jefferis GSXE*, Marin EC*, Stocker RF & Luo L (2001) Target neuron prespecification in the olfactory map of Drosophila. Nature 414: 204-208.

Komiyama T*, Johnson WA, Luo L & Jefferis GSXE* (2003) From lineage to wiring specificity: POU domain transcription factors control precise connections of Drosophila olfactory projection neurons. Cell 112:157-167.

Jefferis GSXE, Vyas, RM, Berdnik D, Ramaekers A, Stocker RF, Tanaka N, Ito K & Luo L (2004) Developmental origin of wiring specificity in the olfactory system of Drosophila. Development 131: 117-130.

Zhu H, Hummel T, Clemens JC, Berdnik D, Zipursky SL# & Luo L# (2006) Dendritic patterning by Dscam and synaptic partner matching in the Drosophila antennal lobe. Nat Neurosci 9: 349-55.

Komiyama T, Sweeney LB, Schuldiner O, Garcia KC & Luo L (2007) Graded expression of Semephorin-1a cell-autonomously directs dendritic targeting of olfactory projection neurons. Cell 128: 399-410.

Hong W, Zhu H, Potter CJ, Barsh G, Kurusu M, Zinn K & Luo L (2009) Leucine-rich repeat transmembrane proteins instruct discrete dendrite targeting in an olfactory map. Nat Neurosci 12: 1542-1550.

Chou YH, Zheng X, Beachy PA & Luo L (2010) Patterning axon targeting of olfactory receptor neurons by coupled Hedgehog signaling at two distinct steps. Cell 142: 954-966.

Sweeney LB, Chou YH, Wu Z, Joo W, Komiyama T, Potter CJ, Kolodkin AL, Garcia KC & Luo L (2011) Secreted semaphorins from degenerating larval ORN axons direct adult projection neuron dendrite targeting. Neuron 72: 734-47.

Hong W, Mosca TJ & Luo L (2012) Teneurins instruct synaptic partner matching in an olfactory map. Nature 484: 201-207.

Joo WJ, Sweeney LB, Liang L & Luo L (2013) Linking cell fate, trajectory choice, and target selection: genetic analysis of sema-2b in olfactory axon targeting. Neuron 78:673-86.

Mosca TJ & Luo L (2014). Synaptic organization of the Drosophila antennal lobe and its regulation by the Teneurins. eLife 3:e03726.

Ward A*, Hong W*, Favaloro V & Luo L (2015). Toll receptors instruct axon and dendrite targeting and participate in synaptic partner matching in a Drosophila olfactory circuit. Neuron 85:1013-1028.

Wu B, Li J, Chou YH, Luginbuhl D & Luo L (2017) Fibroblast growth factor signaling instructs ensheathing glia wrapping of Drosophila olfactory glomeruli. Proc Natl Acad Sci USA 114:7505-7512.

Li H*, Horns F*, Wu B, Xie Q, Li J, Li T, Luginbuhl DJ, Quake SR# & Luo L# (2017) Classifying Drosophila olfactory projection neuron subtypes by single-cell RNA sequencing. Cell 171:1206-1220.

Li J, Guajardo R, Xu C, Wu B, Li H, Li T, Luginbuhl D, Xie X & Luo L (2018) Stepwise wiring of the Drosophila olfactory map requires specific Plexin B levels. eLife 7:e39088.

Xie Q*, Wu B*, Li J, Xu C, Li H, Luginbuhl DJ, Wang X, Ward A & Luo L. (2019) Transsynaptic Fish-lips signaling prevents misconnections between nonsynaptic partner olfactory neurons. Proc Natl Acad Sci U S A. 116:16068-16073.

Li J*, Han S*, Li H, Udeshi ND, Svinkina T, Mani DR, Xu C, Guajardo R, Xie Q, Li T, Luginbuhl DJ, Bing W, McLaughlin CN, Xie A, Kaewsapsak P, Quake SR, Carr SA, Ting AY# & Luo L# (2020) Cell-surface proteomic profiling in the fly brain uncovers new wiring regulators. Cell 180:373-386.

Li H*, Li T*, Horns F, Li J, Xie Q, Xu C, Wu B, Kebschull JM, McLaughlin CN, Kolluru SS, Jones RC, Vacek D, Xie A, Luginbuhl DJ, Quake SR# & Luo L# (2020) Single-cell transcriptomes reveal diverse regulatory strategies for olfactory receptor expression and axon targeting. Curr Biol 30:1189-1198.

Xie Q, Brbic M, Horns F, Kolluru SS, Jones RC, Li J, Reddy AR, Xie A, Kohani S, Li Z, McLaughlin CN, Li T, Xu C, Vacek D, Luginbuhl DJ, Leskovec J, Quake SR#, Luo L# & Li H (2021) eLife 10:e63450.

McLaughlin CN*, Brbic M*, Xie Q, Li T, Horns F, Kolluru SS, Kebschull JM, Vacek D, Xie A, Li J, Jones RC, Lesokvec J, Quake SR#, Luo L# & Li H (2021) Single-cell transcriptomes of developing and adult olfactory receptor neurons in Drosophila. eLife 10:e63856.

* co-first authors. # co-corresponding authors.

Fig. 1-1 – Drosophila olfactory circuit.

Olfactory receptor neurons (ORNs) expressing the same olfactory receptor target their axons (red) to the same glomerulus in the antennal lobe. Projection neuron (PN, green) dendrites also target to single glomeruli, and their axons project to the mushroom body and lateral horn, which mediate learned and innate olfactory behavior, respectively. A single PN is labeled here by the MARCM method (see Section 4). In the wiring of the fly olfactory circuit, about 50 types of ORNs form precise, one-to-one connections with 50 types of PNs at 50 stereotyped glomeruli in the antennal lobe. Images generated by Greg Jefferis, Lisa Marin, and Takaki Komiyama.

Fig. 1-2 – Cellular and molecular mechanisms that assemble the fly olfactory circuit.

(A) PN dendrite targeting. PNs are born in the embryonic and larval stages and specified by chromatin remodeling and transcription factors.They start to extend dendrites at the larval/pupal transition, which creates the proto-antennal lobe. Sema-1a cellautonomously regulates PN dendrite targeting along the dorsolateral–ventromedial axis. Sema-2a/-2b proteins form countergradients to the Sema-1a gradient along the same axis and serve as the extracellular cues that direct this targeting. Subsequently, differential expression Capricious (Caps) instructs the segregation of PN dendrites into discrete glomeruli. (B) ORN axon targeting. ORNs are born in the early pupa. Pioneering ORN axons arrive at the antennal lobe at 18 hours after puparium formation and choose either the dorsolateral or ventromedial trajectories tocircumnavigate the antennal lobe instructed by Sema-2b/PlexinB signaling. ORN–ORN interactions mediated by Sema-1a/PlexinA and ORN–target interactions mediated by Hedgehog (Hh) signaling further constrain ORN axon targeting. (C) ORN–PN matching. Ten-m and Ten-a, two teneurins that are highly expressed in select PN and ORN matching pairs, instruct synaptic partner matching between PNs and ORNs through homophilic attraction. Adapted from Hong & Luo (2014) Genetics 196:17-29.

Fig. 1-3 – Diversity of olfactory projection neuron transcriptomes peaks during the circuit wiring stage.

tSNE plot of transcriptomes of six PN types at three developmental stages. As development proceeds, transcriptomes ofthese six PN types become more similar. 24h and 48h, 24 and 48 hours after puparium formation, respectively. Adapted from Xie et al. (2021) eLife 10:e63450. See also Li et al. (2017) Cell 171:1206-1220. For analyses of ORN transcriptomes, see Li et al. (2020) Curr Biol 30:1189-1198; McLaughlin et al. (2021) eLife 10:e63856.

Fig. 1-4 - Cell-surface proteomes.

Volcano plot comparing developing vs mature cell-surface proteomes in Drosophila olfactory projection neurons. Each dot is a protein (710 total). Proteins known to be involved in synaptic transmission and neural development are colored in red and blue, respectively with the names of a subset indicated. Gray dots represent proteins whose functions had not been previously characterized. A proteome-based screen (gray dots with x,0.5 and y>1) identified 20 new molecules that regulate wiring of the fly olfactory system. Adapted from Li et al. (2020) Cell 180:373-386.