Investigation of the role of polyploidization in glial cells during the development of the drosophila nervous system

Abstract

Organogenesis is a complex process encompassing cell determination, cell differentiation, cell proliferation, and cell size regulation. The proper orchestration of these events ensures that each organ is scaled correctly and can function properly. Polyploidization is a process by which cells increase their DNA content and is used across species to generate large cells. Our lab had previously determined that subperineurial glia (SPG) of the Drosophila melanogaster nervous system become polyploid by both the endocycle and endomitosis. These are two cell cycle variants employed to produce polyploid cells that differ in the latter undergoing some aspects of mitosis but not cytokinesis. Polyploidization of the SPG is critical for blood-brain barrier (BBB) function. Here, we determined that the developmental switch from endocycling to endomitotic SPG occurs in about 70% of SPG in the brain lobes by the second larval instar. The SPG in the ventral nerve cord and peripheral nervous system solely endocycle. We demonstrated that both the Notch signaling pathway and the String Cdc25 phosphatase are critical in determining whether SPG endocycle or endomitose. Experiments manipulating the percentage of cells that are endocycling versus endomitotic highlight key differences between endocycling and endomitotic SPG. We find that endomitotic SPG cells are capable of achieving higher ploidy and cell area values than endocycling cells and are essential to the integrity of the BBB. Strikingly, we find that endocycling SPG within the ventral nerve cord retain the ability to undergo endomitosis when the Notch signaling pathway or the String Cdc25 phosphatase are altered. Further, we showed that a second glial cell type in the peripheral nervous system, wrapping glia (WG), is polyploid and determined that total WG ploidy correlates with nerve length. Interestingly, when WG ploidy was reduced, we found that axonal ensheathment is defective. We also established that the three WG per peripheral nerve differentially contribute to overall ploidy. Axonal ensheathment throughout the entire nerve seems to be dependent on position along the anterior-posterior larval body axis. Finally, we find that reduction of DNA replication components causes reduced WG ploidy only in longer peripheral nerves.