Proper brain development requires neurons to adopt a striking diversity of morphological characteristics that underlie specific functions, and deficits in this process are associated with neuropsychiatric disease (1, 2). Nevertheless, the molecular determinants of neural identity and morphology are poorly understood. The visual system of the fruit fly Drosophila is an excellent venue for investigating this problem, owing to rich genetic tools and well-characterized neuronal cell types with specific morphological characteristics and functions (3-6). T4 and T5 neurons of the Drosophila visual system represent a particularly striking example of divergent morphological characteristics underlying specific functions (7-9). T4/T5 neurons respond to visual motion of light and dark stimuli, respectively, and each is composed of four subclasses (T4a- d and T5a-d) that respond selectively to motion in one of four cardinal directions: front to back (a), back to front (b), upward (c), and downward (d). Remarkably, the dendrites of each class are oriented antiparallel to the preferred direction of motion, and the axons of each class project to distinct corresponding layers of the lobula plate neuropil. How T4/T5 neurons are distinguished from one another at a molecular level, and how this heterogeneity is translated into specific morphologies and functions is poorly understood. High-throughput single cell sequencing was performed to define heterogeneity and discover molecular determinants of identity and morphology in developing T4/T5 neurons. The data reveal multiple distinct clusters of cells with unique gene expression profiles that are enriched in transcription factors and cell surface molecules. This proposal will use these genetic candidates to test the hypothesis that developing T4/T5 neuron classes exhibit distinct molecular identities, and that these identities underlie the unique morphological and functional characteristics of these neurons. Immunohistochemistry in developing T4/T5 neurons will be used to assign candidate expression to specific subclasses, and gene disruption will reveal the functional contribution of molecular heterogeneity to specific morphologies. The experiments will reveal molecular principles governing acquisition of neural identity, morphology and function that are relevant to human nature and neuropsychiatric disease. Training Environment: The proposal will be carried out under the mentorship of Dr. S. Lawrence Zipursky at UCLA / HHMI. Dr. Zipursky's laboratory has discovered fundamental principles of neural identity, morphology and connectivity (10-14), producing leading independent investigators in neurobiology for over three decades. UCLA is a rich intellectual environment with a strong neuroscience community and opportunities for scientific communication, collaboration and professional development. The research training plan will take full advantage of the rich scientific and intellectual resources available in Dr. Zipursky's laboratory and UCLA / HHMI.
Neurons exhibit a striking diversity of unique sizes, shapes, locations and orientations (morphologies), which are essential for proper brain function, and deficits in these characteristics are associated with neuropsychiatric disease. Nevertheless, the genes and molecules that allow neurons to adopt unique morphological features are poorly understood. The proposed study aims to identify the genetic programs that link identity, morphology and function among T4/T5 neurons, which are closely related but functionally and morphologically distinct neuronal cell types in the brain of the Drosophila fruit fly.
