The secretory pathway supplies membrane proteins and lipids essential for cell growth. The development of neuronal dendrites and axons requires large amount of membrane molecules transported by the secretory pathway. However, little is known about the role of the secretory pathway in establishing the architectures of dendrites and axons. From a genetic screen, we isolated dendritic arbor reduction (dar) mutants with reduced dendritic arbors but normal axons of Drosophila neurons. We identified dar2, dar3, and dar6 genes as the homologs of Sec23, Sar1, and Rab1 of the secretory pathway. In both Drosophila and rodent neurons, defects in Sar1 expression preferentially affected dendritic growth, revealing evolutionarily conserved difference between dendritic and axonal development in the sensitivity to limiting membrane supply from the secretory pathway. Whereas limiting ER-to-Golgi transport resulted in decreased membrane supply from soma to dendrites, membrane supply to axons remained sustained. We also show that dendritic growth is contributed by Golgi outposts, which are found predominantly in dendrites. We found that dendritic Golgi outposts are at least in part generated in the soma, excluded from the axon by dynein based transport, and distributed in neuron type specific patterns. Furthermore, we found that a trans-Golgi network localized regulator of secretory trafficking, Rab6, has important roles in dendrite morphogenesis.

Precise dendrite patterning is critical for the wiring and function of the entire nervous system. Proper dendrite morphogenesis determines the properties and strength of synaptic or sensory information a neuron will receive. Furthermore, deficits in dendritic development and function are observed in many neurodevelopmental disorders. Therefore, a better understanding of how the neuron establishes its dendrite morphology during development is necessary and will also provide insights into normal developmental pathways that may be perturbed in certain disorders. Using the Drosophila dendritic arborization (da) sensory neurons, we performed a candidate-based genetic screen to identify molecules that can regulate dendritic arbor morphology. This thesis work focuses on the examination of two candidate mutations identified by genetic screens performed in the lab.

In Chapter 2 we examine the role of the semaphorin signaling in regulating dendrite arbor patterning by restricting dendrites in a 2D space. Loss of semaphorin-2b (sema-2b) led to the detachment of dendrites from extracellular matrix (ECM) during dendrite outgrowth, resulting in failure of dendrites to be confined in a 2D space. Sema-2b proteins are required in epidermal cells, and act through Plexin-B (PlexB) receptor expressed in the neurons. We further show that Tricornered kinase, target of rapamycin complex 2 (TORC2), and integrins act downstream of the Sema-2b/PlexB signaling. Our findings identify a novel role for semaphorins, which are well-known axon guidance proteins, in regulating dendrite patterning, and several new downstream signaling components of semaphorin signaling.

In chapter 3 we examine the role of easily shocked (eas), which encodes a kinase with a critical role in phospholipid phosphatidylethanolamine (PE) synthesis, in dendrite morphogenesis. eas is required cell-autonomously in da neurons for dendrite growth and stability. Further, we show that the level of Sterol Regulatory Element Binding Protein (SREBP) activity increased in eas mutants, and that decreasing the level of SREBP and its transcriptional targets in eas mutants largely suppressed the dendrite growth defects. Furthermore, reducing Ca2+ influx in neurons of eas mutants ameliorates the dendrite morphogenesis defects. Our study uncovers a novel role for EAS kinase and reveals the in vivo function of phospholipid homeostasis in dendrite morphogenesis.

Animals contain mechanisms for interacting with their environment that are exquisitely sensitive to particular modalities. Of all our senses, mechanosensation is the least well understood. The mammalian somatosensory system contains several sensory organs specialized for mechanotransduction, however it is difficult to study for a number of reasons. The da neuron system in Drosophila offers a powerful system in which to study the molecular mechanisms of somatosensation. In this work, the Class III neurons are identified as a rapidly adapting mechanotransducers. In addition, the DEG/ENaC channel Ppk26 is found to interact with Ppk1 in Class IV neurons, where it plays a role in mechanical nociception and locomotion behavior.