Wiring brains correctly requires routing axons and dendrites to appropriate regions, such as layers and columns, to form correct synaptic connections during development. Many neuropsychiatric disorders, such as Down syndrome, Fragile X syndrome and Rett syndrome, have development origins and exhibit abnormal dendritic morphological defects, such as changes in branching numbers and patterns. Dendritic defects could cause neuronal connectivity defects, which likely underline neurological and cognitive deficits. It remains unclear, however, how genetic disorders lead to dendritic patterning defects during development, which in turns lead to erroneous connections and functional deficits in adults. Our group uses the Drosophila optic lobe neurons as a model to study dendritic development and neural circuit assembly in the central nervous system. Similar to vertebrate cortex and retina, the Drosophila optic lobe is organized in columns and layers, suggesting that the fly visual neurons and vertebrate cortex neurons face similar challenges in routing their dendrites to specific layers and columns during development. Furthermore, the fly visual neurons have unique advantages. First, the medulla neurons extend dendrites to form synapses in a lattice-like structure. Second, specific subtypes of medulla neurons can be specifically labelled and their genetics manipulated at the single-cell resolution. Third, the synaptic circuits have been characterized at the ultrastructural level and can be analyzed at the light microscopic level. Forth, functional deficits can be fully characterized using behavioral assays and functional imaging assays. We have further developed a number of novel techniques to generate high-resolution images, to standardize and to compare dendritic patterns, and to visualize synaptic connections at the light and electron microscopic levels, thus facilitating phenotypic analyses. We have carried out two genetic screen to identify molecular determinants that control dendritic patterning of Tm20 and Dm8 neurons. We focused on four types of dendritic developmental defects: (1) the initiation of main dendritic branches; (2) the dendritic projection directions; (3) the layer-specific targeting of dendrites; (4) dendritic field sizes. From the genetic screens, we have identified adhesion receptors, morphogen receptors, signaling molecules, and cytoskeletal regulators that are cell-autonomously required in Tm20 and/or Dm8 neurons for proper dendritic development. The RNAi-screen identified families of cadherins and cadherin-like receptors that are required for proper initiation of dendritic branches and receptive field sizes. The classical cadherin N-cadherin is required cell-autonomously in Tm20 neurons for layer-specific initiation of main dendritic branching points. Unlike wild-type Tm20 neurons, which extended most dendritic branches from one or two primary branching nodes located in the medulla M3 layer, Ncad mutant Tm20 neurons initiated the main dendritic branches in the M2 layers. The layer shift of the main branching nodes in Ncad mutant Tm20 is further compounded with an alteration of layer-specific targeting of their dendritic arbors and their planar projection directions. Interestingly, the total dendritic length was unaffected. This suggests that Ncad mutation specifically affects the initiation of primary dendritic branches rather than branch trimming. The loss-of-function mosaic screen identified two pathways that regulate the sizes of dendritic trees. We have previously demonstrated that the TGF-beta/Activin signaling pathway negatively controls the sizes of the dendritic fields of Tm20 and Dm8. Mutant Tm20 lacking Activin signaling components, such as the receptor Baboon and the downstream transcription factor Smad2, elaborated an expanded dendritic tree, spanning several medulla columns. Morphometric analyses revealed that baboon and smad2 mutations significantly reduced dendritic termination frequency but not branching frequency. Using a modified GRASP method we developed, we found that the expanded dendritic tree of mutant Tm20 forms aberrant synaptic contacts with several neighboring R8 photoreceptors. RNAi-knock-down of Activin in R7 and R7 further showed that Activin derived from photoreceptors R7 and R8 act in short ranges on R7s and R8s respective synaptic targets, Dm8 and Tm20. Recently, we found that the insulin signaling pathway positively regulates the dendritic tree size of Dm8. Mutant Dm8 neurons lacking insulin receptor or the downstream signaling components TOR (target of rapamycin) or Rheb have a small dendritic tree. On the other hand, mutant Dm8 neurons lacking the negative regulator of the insulin signaling pathway, Pten (Phosphatase and tensin homolog) or TSC1 (Tuberous Sclerosis 1), have an abnormally expanded dendritic tree, as compared to the wild-type. Mis-regulation of TOR signaling, collectively called mTORopathies, has been implicated in several focal malformations of cortical development (MCD) subtypes associated with epilepsy and dendritic morphological defects. However, the dissection of TOR signaling pathways in complex mammalian nervous systems has been difficult. The Dm8 and Tm20 dendritic development system we developed thus provides a speedy way to dissect the complex phenotypes of the TOR pathway and to determine the cell-autonomous functions of TOR signaling at the single cell resolution.
