Fruit fly motoneuron dendrites are not essential for basic functions
Dendrite architecture determines fine control of motor abilities
Dendrites are branched projections of nerve cells that are involved in the reception of synaptic input. An estimated 100 billion neurons in the human brain form about 100 trillion synapses onto 150,000 kilometers of dendritic cable. One fundamental problem that is being considered in basic neurobiological research is why nerve cells have so many dendrites. The standard explanations range from providing enough surface area for synaptic input to the formation of highly specific compartments for molecular signaling and neuron information processing. Structural defects in dendrites are linked to various brain disorders such as autism, Alzheimer's, and schizophrenia. "However, it is often unclear whether defects in dendritic structure are the cause or the consequence of impaired cerebral functioning," clarified Professor Carsten Duch of the Institute of Zoology of Johannes Gutenberg University Mainz (JGU).
Duch's work group at Mainz University is investigating dendritic structures and functions using the fruit fly Drosophila as a model organism. When analyzing the role of dendrites, the experimental challenge is to abolish them selectively in identified Drosophila wing motoneurons without affecting other properties of these specific neurons or others. This enables the researchers to determine exactly how this manipulation influences the ability of Drosophila to carry out activities. Using genetic stratagems, the biologists have managed to do exactly that in their most recent latest study on Drosophila motoneurons.
"To our surprise, we discovered that these motoneuron dendrites are actually dispensable for synaptic targeting, qualitatively normal neuronal activity patterns during behavior, and basic behavioral performance," explained Dr. Stefanie Ryglewski and Dr. Dimitrios Kadas, the two primary authors of the study. However, detailed physiological investigations and behavioral experiments have shown that removing the dendrites compromises the functional fine-motor control exercised by these neurons. This, in turn, impairs the ability of Drosophila to perform sophisticated motor behaviors, such as flight altitude control and switching between song elements during courtship behavior.
The team working with Professor Carsten Duch has thus uncovered the first direct evidence that complex dendritic architecture is a critical factor that controls the behavioral processes defined by evolution that are vital for reproduction and survival. The study also provides a possible explanation for why during evolution the maintenance of complex dendritic structure is under high selective pressure. In addition, the extent of Drosophila motoneuron impairment increases in parallel with the extent of dendritic damage. This kind of correlation between the severity of dendrite defects and the degree of neural deficiency can also be observed in association with increasing structural damage in progressive neurological disorders.