Analysis of Dscam1 diversity in regulating dendritic morphology of Lobula Plate Tangential Cells

In arthropods like Drosophila, Down syndrome cell adhesion
molecules (Dscam1) exhibit enormous molecular diversity. A single
Dscam1 gene encodes a large superfamily of neuronal cell
recognition proteins that control neuronal outgrowth and anatomy. A
comparable function is exhibited by the vertebrates DSCAMs of which
only few isoforms exist. However, it is largely unknown, if and how
this function of Dscams affects neuronal function and the control
of behavior by the nervous system. In this thesis, I employed an
arsenal of genetic techniques to perturb the expression level of
Dscam1 isoforms in directionally selective Lobula Plate Tangential
Cells (LPTCs). LPTCs of the Vertical (VS) and the Horizontal System
(HS) were chosen as a model system because of their well-documented
anatomy, role in information processing and behavior. Though, only
little is known about the developmental mechanisms and molecular
factors controlling the morphogenesis and wiring of these cells.
The central aim of my study thus is to reveal a possible role of
Dscam1 in the growth and development of the complex dendrites of in
particular HS cells. Furthermore, my work aims at establishing a
novel model system for integrated studies on the development and
function of LPTCs by genetic manipulations of Dscam1 expression. My
results demonstrate that Dscam1 is expressed broadly in the fly
visual system including HS-cells (immunolabeling of the conserved
intracellular domain). Loss of Dscam1 function and reduced isoform
diversity consistently elicited misrouting and self-crossings of
neurites in LPTC dendrites. In contrast, misexpression of selected
single Dscam1 isoforms caused a severe reduction in the size and
branching complexity of LPTC dendrites. The dendritic
gain-of-function phenotype (ectopic expression of the Dscam1
isoform 11.31.25.1) was strongly dependent on the time of onset of
misexpression during development. These results demonstrate that
Dscam1 contributes to the development of LPTC dendrites. This
system can now be used to (A) address a possible role of Dscam1 in
the function of neurons and circuitries and (B) to address the
interplay of anatomy and function of LPTC dendrites. In further
side projects I aimed at the development of additional genetic
tools for the investigation of the role of LPTCs in behavior and
for studies on the wiring of LPTCs to the presynaptic circuitry. I
established a heat-shock protocol for the ablation of specified
LPTCs by RicinA expression and I generated a fly line for the
expression of TN-XXL (a genetically encoded calcium biosensor) in
small cell clusters or individual cells. Finally, I participated in
efforts to establish a virus based retrograde labeling method in
Drosophila.