Compared to axon guidance, relatively little is known about synaptic target selection mechanisms2-4
. Among the notable examples, the graded expressions of vertebrate EphA and Ephrin-A instruct the topographic targeting of retinal ganglion cell axons4,33-35
. Chick DSCAMs and Sidekicks promote lamina-specific arborization of retinal neurons36
Capricious promotes target specificity of photoreceptor and motor axons16,37-39
. C. elegans
SYG-1 and SYG-2 specify synapse location through interaction between pre-synaptic axons and intermediate guidepost cells40
. However, it is unclear whether any of these molecules mediate direct, selective interactions between individual pre- and post-synaptic partners. Indeed, in complex neural circuits, it is not clear a priori
whether molecular determinants mediate such interactions. For example, the final retinotopic map is thought to result from both Ephrin signaling and spontaneous activity41,42
. Mammalian ORN axon targeting involves extensive axon-axon interactions through activity-dependent and independent modes43,44
, with minimal participation of postsynaptic neurons identified thus far.
Here, we show that Teneurins instruct PN-ORN matching through homophilic attraction. Although each glomerulus contains many synapses between cognate ORNs and PNs, these synapses transmit the same information and can be considered identical with regard to specificity. Thus, Teneurins represent a strong case in determining connection specificity directly between pre- and post-synaptic neurons. We further demonstrate that molecular determinants can instruct connection specificity of a moderately complex circuit at the level of individual synapses.
Our study reveals a requirement for PN-ORN attraction in the stepwise assembly of the olfactory circuit. PN dendrites and ORN axons first independently target to appropriate regions using global cues, dendrite-dendrite and axon-axon interactions8,9,12-14
. These initial, independent dendrite and axon targeting are eventually coordinated in their final one-to-one matching. We identified Teneurins as the first molecules to medicate this matching process, through direct PN-ORN attraction. Our analyses have focused on a subset of ORN-PN pairs involving trichoid ORNs45
, including Or67d/Or88a/Or47b that are implicated in pheromone sensation46
. The partially overlapping expressions of Teneurins in other PN and ORN classes (, S4
) suggest a broader involvement of Teneurins. At the same time, additional cell-surface molecules are also needed to completely determine connection specificity of all 50 PN-ORN pairs.
Teneurins are present throughout Animalia
(). Different vertebrate Teneurins are broadly expressed in distinct and partially overlapping patterns in the nervous system18
. Teneurin-3 is expressed in the visual system and is required for ipsilateral retinogeniculate projections47
. Our study suggests that differential Teneurin expression may play a general role in matching pre- and post-synaptic partners. Indeed, high-level Ten-m is involved in matching select motoneuron-muscle pairs26
. Furthermore, Teneurins also trans-synaptically mediate neuromuscular synapse organization26
. This suggests that the synapse partner matching function of Teneurins may have evolved from their basal role in synapse organization. Interestingly, synaptic partner matching only involves homophilic interactions (this study and ref. 26
), whereas synapse organization preferentially involves heterophilic interactions26
. This could not be fully accounted for by different strength of their homophilic and heterophilic interactions in vitro
(). Indeed, while heterophilic interactions occur in vitro
(), heterophilic overexpression of Ten-m and Ten-a in AM29 ORNs and Mz19 PNs did not produce ectopic connections (not shown). Thus we speculate that these dual functions of Teneurins in vivo
may engage signaling mechanisms that further distinguish homophilic versus heterophilic interactions.