Final Report Summary - GSL IN DEVELOPMENT (The role of glycosphingolipids in development)
The initial aim of our project was to understand the role of glycosphingolipids in development. During the course of this study, we have identified that one of the enzyme involved in the synthesis of core glycosphingolipids, brainiac (brn), was important in the biology of the stem cell niche of the testis in Drosophila melanogaster males. Indeed, these males presented an abnormal maintenance of the stem cells forming their testis niche. This seminal finding led us to focus more precisely on the testis stem cell niche homeostasis in order to understand with precision what could be the influence of brn in the signalling network underlying the regulation of the maintenance and the activity of the testis stem cell niche.
The testis stem cell niche is localized at the apical tip of the testis and formed by three main populations of cells. Some post-differentiated cells, the hub cells (HC), form together a structure displaying a hub architecture around which are anchored two populations of stem cells: the germ stem cells (GSC) which give rise to the sperm and the somatic stem cells (SSC). SSC give rise to cyst cells that accompany and support GSC over spermatogenesis and can replenish HC over age. In addition, both HC and SSC can participate to the maintenance and the regulation of the activity of GSC. The relative contribution of SSC and hub cells to GSC regulation remains however a matter of debate. Finally, GSC can control SSC differentiation. Hence given the tight functional relationship existing between HC, SSC and GSC, we have decided in our work to consider that all these three populations of cells form the testis stem cell niche.
In order to describe quantitatively the functional state of the testis stem cell niche, we have developed a novel analytical approach consisting in first, counting precisely each of the cell populations forming the testis niche by taking advantage of our ability to identify all HC, SSC and GSC cells; second, applying a Pearson correlation matrix analysis to these data in order to describe the existence of any correlation between two cellular populations at the single testis level. High correlation values between two cellular populations, HC and GSC for example, can indeed be interpreted as a functional niche interaction between these two types of cells. By using this strategy, we have been able to describe quantitatively the variation and the dynamic of the activity of the testis stem cell niche in various environmental and physiological conditions, in other words the phenotypic plasticity of the testis stem cell niche.
Importantly, during this characterization, we realized that Wolbachia pipientis, an alphaproteobacteria, which is a natural endosymbiont of D melanogaster in the wild, affected significantly the testis niche homeostasis. By precisely analyzing the influence of this bacteria on the homeostasis of the testis stem cell niche in wild type males, we have demonstrated that Wolbachia pipientis likely affected the relative contribution of HC and SSC to GSC maintenance. In addition, the infection by the bacteria was associated to a better maintenance of both somatic and germinal stem cells during the aging of Wolbachia-infected males. These results suggest that Wolbachia infection might promote a better fitness of the infected males. This manuscript is currently in revision.
Conclusions and Impact of the work:
The homeostasis of self-renewing tissues is dependent on the appropriate regulation of stem cells by their microenvironment. Understanding how environmental and physiological changes impact on the activity of stem cells is a key issue. During our IRG granting period, we studied the impact of such changes on the stem cells of the Drosophila testis. Our study has allowed us to highlight the high degree of phenotypic plasticity of this niche and to provide a useful analytical framework to study in the future the impacts of environmental, physiological and genetic perturbations on the homeostasis of a stem cell niche.
Our work has in addition unveiled a previously unknown impact of the Wolbachia endosymbiont on Drosophila testis niche. Wolbachia pipientis is a common endosymbiont in many different arthropod and worm species on the planet. Recently, Wolbachia has become became famous for conferring a resistance of its host to many different parasites and virus, among which those responsible for Malaria and the lethal Dengue fever. By opening a new window on the impact of Wolbachia on the physiology and the tissue homeostasis of its hosts, our work, will hopefully inspire new research avenues allowing the scientific community to understand how this bacteria manipulates the immune system of its hosts and fortunately impair their ability to spread lethal human pandemic diseases.