Drosophila melanogaster: a model system to study centriole elimination and basal body dynamics

The centrosomes play an essential role in cell and tissue homeostasis, therefore, their structure, function, and number are highly regulated to ensure natural organisms development through the assembly of a multiplicity of protein complexes. Since the organization and integrity of the centrosome depend on its centrioles and pericentriolar material (PCM), understanding the dynamics of these organelles is crucial to decipher the centrosome behaviour. To date, we have a fairly detailed knowledge of the centriole composition and structure and also of the process of duplication and centrosomal maturation. Something is understood about the process of centriole elimination during gametogenesis, but very little is known about how centrioles are eliminated in post-mitotic differentiated cells. During the development of the Drosophila eye, the centrioles of the differentiating retinal cells do not recruit γ-Tubulin, suggesting that they are unable to organize functional microtubule-organizing centers (MTOCs). Consistent with this hypothesis, this study shows that in Drosophila third instar larvae Cnn and Spd-2, proteins that allow γ-Tubulin recruitment, and DPlp, which is involved in the organization of the pericentriolar material, are not accumulated by centrioles of eye imaginal disc cells. Despite the loss of these essential components of the pericentriolar material, the centrioles are structurally intact and can recruit Asl and ANA-1. Usually, the accumulation of Asl and ANA-1 allows the daughter centrioles to acquire the motherhood condition. Indeed, mother centrioles accumulate properly Plk-4; however, they are not able to duplicate. These findings show that, in this model, the accumulation of Plk-4 is not sufficient to allow centriole duplication. During the progression of pupal development, the centriole number progressively decreases, and structural defects can be observed. These phenotypes suggest that during Drosophila eye development centriole elimination begins with the loss of the structural integrity, rather than with the PCM reduction as occurs in other models. Furthermore, Asl, ANA-1 and Sas-4 are still detectable, indicating that these proteins by themselves are not able to ensure the maintenance of centriole integrity. Among the essential cellular functions played by centrioles, there is their ability to act as basal bodies to nucleate the axoneme, the supporting structure of cilia and flagella, which perform crucial cellular functions such as signal transduction and cell motility. Given the critical role of centrioles and cilia in cell physiology, mutations in numerous centriolar proteins cause various disorders, including microcephaly, dwarfism and ciliopathies. Therefore, it is crucial to understand better the mechanisms that regulate the dynamics of centrioles and cilia. In this study, the cilia of Drosophila melanogaster type I sensory neurons have been analysed, to understand the role played by the centriolar proteins Klp10A, Cnb, Gorab and Rcd4 in the dynamics of centrioles and cilia. In Drosophila wild type sensory neurons, Klp10A (Kinesin-like protein 10A), a member of the kinesin-13 family, is located in the distal part of the transition zone (TZ), just above the UNC–GFP signal. This study shows that mutations in klp10A result in substantial structural defects of sensory neurons such as the over elongation of both centrioles in opposite directions. It has also been observed that the extensions of both centrioles, called proximal and distal basal bodies, show doublets surrounded by electron-dense material and short lateral projections as found in the control TZ. Therefore, the elongated distal regions of the centrioles in klp10A mutants may be equivalent to a TZ. The phenotype observed in klp10A mutant is deeply different from that observed in sensory neurons of mutants for other TZ proteins that are limited to the proximal portion of the TZ. This suggests that Klp10A could be a core component of the ciliary transition zone in Drosophila, specifically associated with the distal region of the TZ where it plays an essential role in centriole elongation and the assembly and maintenance of the ciliary axoneme. Centrobin (Cnb) is a centrosome-associated protein that localizes specifically at the daughter centrioles. It has been shown that a cnb mutation makes the daughter centrioles, called PBBs in this model, able to act as distal basal bodies (DBBs) to nucleate supernumerary axonemes. This is confirmed by the present study performed on a different cnb mutant strain, suggesting that Cnb acts as a negative regulator of ciliogenesis. Recently a new centriolar protein required for centriole duplication, called Gorab, has been discovered in Drosophila melanogaster. The cnb-gorab double mutant sensory neurons analysed in this study, show a stronger centriole reduction compared to the single gorab mutant. Consequently, the number of cilia is also severely affected. These findings suggest that in the cnb-gorab mutant, the centriole duplication fails before the basal body formation. Recent works have identified the human protein called PPP1R35 (Rcd4 in Drosophila - Reduction in Cnn dots 4), that is involved in centriole-to-centrosome conversion (CCC) and centriole elongation. Here we demonstrate that rcd4 mutant sensory neurons show a severe centriole and cilia reduction, accompanied by centriolar fragmentation. This suggests that Rcd4 could be involved in the CCC similarly to its human counterpart.