Dendritic spines and structural plasticity in Drosophila

The morphology of dendrites is important for neuronal function and
for the proper connectivity within neuronal circuits. The often
very complex shape of dendritic trees is brought about by the
action of many different genes throughout development. Moreover,
neuronal activity is often involved in refining synaptic
connections and shaping dendrites. Aiming at a better understanding
of the interplay between genes and neuronal activity during
dendrite differentiation I was trying to identify suitable neurons
in the Drosophila central nervous system. Describing the morphology
and cytoskeletal organization of a group of visual interneurons
involved in motion processing I provided evidence that the
dendrites of these neurons do bear small protrusions that share
essential characteristics with vertebrate spines. Vertebrate spines
received a lot of recent attention because neuronal activity can
induce lasting changes in their morphology even in the adult. These
morphological changes are believed to be cellular correlates of
learning and memory. The observation of similar structures in flies
raised the possibility to study structural plasticity in a
genetically accessible model organism. Experience-dependent
alterations in the volume of a region in the insect brain, called
mushroom body calyx, have been shown. The calyx is known to contain
the dendrites of olfactory interneurons, Kenyon cells, which are
known to be required for the retrieval of olfactory memories in
flies. I wanted to address if morphological rearrangements of the
dendrites of these cells could underlie the experience-dependent
changes in calycal volume. Kenyon cell dendrites and their
presynaptic partners are known to form synaptic complexes, called
microglomeruli, throughout the calyx. My results help refining the
anatomical description of these structures. These findings are
important to understand how olfactory experience is represented in
the fly brain and how olfactory memories might be formed. Moreover,
I developed a computer algorithm to quantitatively describe the
morphology of these microglomeruli in an automated manner. Thereby,
I found indications for morphological rearrangements of calycal
microglomeruli during the first days of the adult life of
Drosophila. I could show that olfactory experience is not required
for these morphological alterations. My findings provide the basis
for ongoing attempts to study the influence of neuronal activity on
the dendritic morphology of Kenyon cells in more detail.