2026 Theses Doctoral

The roles of neural activity in Drosophila mechanosensory circuit assembly

Qi, Nova

The assembly of functional neural circuits requires the precise coordination of axon growth, target selection, and structural refinement during development. While molecular guidance programs establish initial patterns of connectivity, neuronal activity plays a critical role in shaping mature circuit architecture. However, the mechanisms by which developmental activity patterns are translated into stable structural and functional organization of sensory circuits remain incompletely understood.

In this thesis, I used Drosophila larval chordotonal (ch) neurons as a model system to investigate how neuronal activity regulates sensory axon terminal development and integration into mature mechanosensory circuits. Ch neurons are mechanosensory neurons that detect vibration and body movement and project axons into the ventral nerve cord (VNC), where they form stereotyped sensory terminal arbors. These neurons also exhibit patterned spontaneous activity during embryogenesis prior to the onset of sensory-driven input, suggesting that early physiological activity contributes to developmental circuit assembly. Their highly stereotyped organization, developmental accessibility, and conserved mechanisms of sensory circuit assembly make ch neurons a powerful system for investigating how neuronal activity shapes developing neural circuits.

I found that neuronal activity regulates the spatial organization and structural maturation of ch axon terminals within the larval VNC. Analysis across larval development revealed that ch axons undergo a transient phase of exploratory growth characterized by increased medial extensions during intermediate developmental stages followed by refinement into mature terminal arbors. Manipulation of neuronal activity altered the timing and extent of this process, with silencing reducing exploratory growth and chronic activation promoting persistent medial extensions.

To characterize these structural changes at single-cell resolution, I combined single-cell labeling with a machine learning-based morphological analysis pipeline. This analysis identified a developmental transition from immature, filopodia-rich terminal arbors to elongated and spatially refined mature structures. Silenced neurons failed to complete this transition and retained immature terminal morphology at late developmental stages, while activated neurons exhibited reduced terminal size and altered branch organization. Together, these findings demonstrate that neuronal activity coordinates both the positioning and maturation of sensory axon terminals during development.

To determine how these structural changes influence circuit function, I examined mechanosensory behaviors mediated by ch neurons. Perturbation of neuronal activity during development altered the magnitude and maturation of behavioral responses to vibration stimuli, indicating that activity-dependent structural maturation contributes to the emergence of appropriate sensory circuit output.

Finally, I identified a previously uncharacterized neuron degradation phenotype associated with long-term expression of optogenetic channelrhodopsins in developing somatosensory neurons. Expression of the red-light activated channel Chrimson in the absence of all-trans retinal (ATR) produced progressive axon and dendrite loss, revealing an important experimental limitation associated with chronic optogenetic manipulation in developing circuits. Altogether, this work provides insight into the mechanisms by which developmental neuronal activity shapes sensory circuit organization, maturation, and functional output.