Afferent proprioceptive signaling in Drosophila larvae

ABSTRACT Proprioception is the sense that allows animals to monitor their body position and movements; proprioceptive deficits lead to severe challenges in moving and maintaining posture. Proprioceptive feedback passes from sensory neurons through two central nervous system (CNS) pathways: local pathways targeting motor circuits, where proprioception adjusts ongoing movements, and projection pathways to the brain, where proprioception is used to learn and plan future actions. The fundamental differences between projection and local pathways remain largely unknown. Our long-term goal is to understand proprioceptive circuit activity during natural animal behavior, emphasizing the roles played by genetically-defined cell types, which will facilitate the discovery of stem-cell-based therapies for injury and proprioceptive dysfunction. This proposal's objective is to characterize fundamental differences between projection and local pathways. We focus on second-order neurons, CNS neurons that receive direct input from proprioceptive sensory neurons of each pathway. We will specifically address the following questions: does the brain receive minimally processed stimulus information or integrated representations of specific stimulus features? Is information presented to the brain in a behavioral state-dependent manner? Does the brain receive privileged types of information in comparison to local circuitry? We use Drosophila larva as a highly tractable model to study proprioception. This proposal deploys two major technical innovations: CRASH2p microscopy, which allows for volumetric imaging of neural dynamics in intact, freely moving, and behaving larvae, and connectomics, which allows for comprehensive reconstruction of synaptic connections between second-order neurons and their synaptic partners. Based on preliminary data, we will test the central hypothesis that local and projection second-order proprioceptive neurons differentially integrate and process naturally occurring self-movement stimuli. We test this hypothesis using complementary in-depth (functional, Aim 1) and in-breadth (anatomical, Aim 2) experimental strategies. The proposed research is significant because it will provide two advances that remain, to date, out of reach in other models. First, it will provide a comprehensive anatomical understanding of the diversity of second-order proprioceptive neurons and the networks in which they are embedded. Second, it will produce first-of-its-kind knowledge of the activity of second-order proprioceptive neurons in intact animals performing multiple behaviors and determine the role of a specific type of proprioceptor in shaping that activity. Thus, our work is expected to provide a new conceptual framework for how diverse second-order neurons integrate and process proprioceptive information and how the brain senses proprioceptive stimuli.