|Ph.D Student||Dunayevskyy Adel|
|Subject||Programmed Cell Death and Cell-Fate Determination in the|
Proprioceptive Organs of Drosophila Melanogaster
|Department||Department of Medicine||Supervisor||Professor Adi Salzberg|
|Full Thesis text|
Coordinated locomotion of Drosophila larvae depends on sensory input from stretch receptive proprioceptors named chordotonal organs. The chordotonal organs sense relative displacement of body parts and transmit this information to the central nervous system. Each abdominal hemisegment of the larva contains eight chordotonal organs, five of which are clustered together and constitute collectively the pentascolopidial organ (LCh5). Each of the five chordotonal organs within the LCh5 cluster originates from a single precursor that goes through four asymmetric cell divisions to generate five cells with different identities: a neuron, a scolopale, a cap, a ligament and a cap-attachment (CA) cell. However, whereas the LCh5 cluster contains five neurons, five scolopale cells, five ligament cells and five cap cells, it contains only two CA cells. The reason for this discrepancy and the mechanism underlying the loss of three CA cells remained elusive until now.
Here we show that three of the five initially developed CA cells undergo cell death during early stages of LCh5 development. We further show that, normally, these cells die by classic apoptosis, as suggested by the presence of apoptotic cells in the immediate surroundings of the surviving CA cells and by the survival of the doomed CA cells in embryos expressing a pan-caspase inhibitor, p35. If, however, the apoptotic pathway is blocked prior to caspase liberation, the CA cells are eliminated by combined action of the Bcl-2 proteins and autophagic machinery. Phenotypic analysis of larvae in which CA cell-death was blocked revealed the importance of CA cell elimination for proper morphogenesis and function of the LCh5 organ.
Another question addressed in this study is how the different LCh5 cells acquire their unique properties. Specifically, we focused on the role of the prospero (pros) gene in this process. We demonstrate for the first time that pros influences cell-fate determination in the chordotonal lineage. Within the LCh5 organ pros’ expression is restricted to the scolopale cell. We show that in pros null mutants scolopale cells up-regulate the expression of cap-cell specific genes (e.g. delilah (dei)), but they still manifest some scolopale-specific characteristics such as scolopale rods. In addition, cap cells that are forced to express pros adopt a scolopale-like phenotype consisting of down-regulation of cap cell-specific genes along with up-regulation of scolopale cell-specific determinants. Thus, pros is an important cell-fate determinant whose activity is critical for normal differentiation of scolopale versus cap cell identity.
The adjustment of CA cell numbers and the establishment of correct cell identities and cell morphologies are both critical for organogenesis of functional proprioceptors.