Regulation and function of Rootletin - a gene differentially expressed in Drosophila sensory neurons

Abstract

Drosophila melanogaster is a widely used and efficient genetic model to study nervous system development. The conservation of many genes from Drosophila to vertebrates and a short reproduction cycle makes the fruitfly a great tool for providing insight into crucial events in nervous system formation. In studying the development of the sensory nervous system, Drosophila also provides a model for understanding the formation and function of structurally diverse cilia. Cilia are hairlike organelles present throughout our bodies and responsible for many processes such as chemo, mechano, and thermosensation, fluid movement, hearing and fertility. In Drosophila the only somatic ciliated cells are the Type I sensory neurons in which a cilium forms the sensory dendrite. There are more than two diverse subtypes of the ciliated sensory neurons and the mechanism by which this diversity is achieved remains unclear. The mechanism of ciliated sensory neuron differentiation was hereby studied on an example of a differentially expressed ciliary gene - CG6129 - a Drosophila orthologue of human Rootletin, a main protein components of ciliary rootlets. CG6129 expression is specific to the ciliated cells and exhibits so called chordotonal-enriched pattern - a strong and permanent expression in the chordotonal subtype of type I neurons and weaker and transient expression in the external sensory subtype. I have shown that CG6129 knock-down causes severe disruption of the chordotonal organs function without any obvious change in the structure of the cilium, other than the lack of ciliary rootlet. The function of the external sensory subtype was only slightly affected which further highlights the difference between the two types of ciliated sensory organs. The fact that CG6129 is differentially expressed in the two subtypes of the Drosophila ciliated sensory neurons suggests that the genes involved in the formation of various cilia are differentially regulated. I have shown that CG6129 is regulated by the two well known ciliary transcription factors - RFX and fd3F (distant homologue of Foxj1). Of the two enhancers found the early-to-late enhancer is almost entirely dependent on RFX and not on fd3F while the late enhancer is dependent on both fd3F and RFX. The fact that there is some residual CG6129 expression in the absence of both RFX and fd3F suggests involvement of another regulator that may contribute to the cilia diversity. Zmynd10 is a recently characterised ciliary gene that is involved in the axonemal dynein arms assembly. Mutations in human Zmynd10 cause primary ciliary dyskinesia (PCD) and Drosophila Zmynd10 mutants have immotile cilia that lack dynein arms. Due to the presence of specific protein domains Zmynd10 has been suggested to act as a transcriptional regulator. I have shown that the transcript levels of CG6129 and other ciliary genes are reduced in the Zmynd10 mutant. This implies that Zmynd10 may regulate ciliary genes on a transcriptional or post transcriptional level and may contribute to the regulatory network governing ciliogenesis.