Title: Axon-axon and axon-target interactions underlying somatosensory circuit assembly in Drosophila
Description: Sensory axons from functionally related neurons often project to similar regions in the central nervous system (CNS). Various cell-cell interactions and activity-dependent mechanisms contribute to the formation of these arrangements, but it remains unclear how they ultimately influence circuit wiring and function. I examined mechanisms of somatosensory circuit assembly in Drosophila. In larvae, class III (cIII) and class IV (cIV) dendritic arborization neurons detect gentle touch and noxious stimuli, respectively. Sensory axons travel together to the CNS and terminate in the ventral nerve cord (VNC). Previous work showed that within the VNC, touch and nociceptive axons sort into adjacent layers and make modality-specific synaptic connections with a population of nociceptive interneurons. The organization of somatosensory afferents is similar in insects and vertebrates, but mechanisms underlying somatosensory circuit formation are not well understood. I identified a role for axon-axon interactions in modality-specific targeting and connectivity of touch neurons.

Ablation of nociceptors resulted in touch neurons extending axons into the nociceptive region and expanding connectivity with nociceptive interneurons. By contrast, nociceptor axon targeting was not noticeably impacted by touch neuron ablation, suggesting that axon interactions act hierarchically to influence axon targeting. To understand how axon sorting emerges during development, I developed a method to perform time-lapse imaging of sensory axons during targeting. Preliminary results suggest that sensory axons arrive in the ventromedial neuropil sequentially based on target layer. I show that nociceptors also impact the transduction of touch stimulus. Whereas touch neuron activation normally elicits behaviors associated with touch stimulus, either ablation or silencing synaptic transmission in nociceptors led to behaviors associated with noxious stimuli. These results point to a possible role for neural activity in touch and nociceptive circuit wiring and function. In support of this, manipulating activity in touch or nociceptive neurons disrupted axon patterning.

Additionally, I present a role for Down syndrome cell adhesion molecule 2 (Dscam2) in regulating connectivity between synaptic partners in the nociceptive circuit. Previous work showed that alternative splicing of Dscam2 generates two isoforms. I found that synaptic partners in the larval nociceptive circuit express complementary isoforms. Regulated alternative splicing of Dscam2 is required for robust nociceptive behavior and proper nociceptive axon patterning. Furthermore, forcing synaptic partners to express a common isoform resulted in nociceptive axon targeting defects. I propose that regulated expression of Dscam2 isoforms may be a mechanism to restrict connectivity to select groups of neurons. Taken together, these data support roles for axon-axon, axon-target, and possible activity-dependent mechanisms in somatosensory circuit assembly.