Delta family protocadherins contribute to protoglomerular targeting of olfactory sensory neuron axons in the olfactory bulb.

To understand how neural circuits are assembled, it is essential to identify and characterize the axonal guidance cues and receptors that determine the axonal trajectories and connections between neurons. We performed single-cell RNA sequencing of olfactory sensory neurons from zebrafish to identify candidate axonal guidance-related genes that are differentially expressed according to sensory axon target location in the olfactory bulb. Among the candidates we identified were several members of the non-clustered delta-protocadherin family of adhesion molecules. We found that two members of the delta1-protocadherin family, pcdh7b and pcdh11, are most highly expressed in sensory neurons that project to a specific identifiable neuropil in the early olfactory bulb called the DZ protoglomerulus. Knocking down either one of these protocadherins impairs the ability of sensory axons to terminate within the DZ protoglomerulus. Knockdown does not affect the ability of other sensory axons from terminating normally in a separate neuropil called the CZ protoglomerulus. In contrast, two members of the delta2-protocadherin family, pcdh10b and pcdh17, are most highly expressed in sensory neurons that project to the CZ protoglomerulus. Knocking down pcdh10b induces ectopic terminations of CZ projecting sensory axons. Knocking down pcdh17 induces substantial ectopic axonal trajectories and impairs CZ projecting sensory axons from finding and terminating in the CZ protoglomerulus. Knockdowns of either pcdh10b or pcdh17 do not affect DZ projecting sensory axons. We conclude that delta1-protocadherins help DZ projecting sensory axons enter and remain within the DZ protoglomerulus, while delta2-protocadherins help CZ projecting sensory axons navigate to the CZ protoglomerulus.