The influence of tissue organization on the generation of TGF-ß signals and the function of stem cell niches in the Drosophila melanogaster germline.
Zusammenfassung der Projektergebnisse
The aim of the study was to elucidate how tissue organization shapes stem cell niches by controlling the spreading of intercellular signals. Previous work in several systems had suggested that adhesion and signaling processes between niche and stem cells are functionally linked. It had therefore been proposed that tight spatial regulation of niche signaling is achieved via putative adhesion based “stem cell niche synapses”. We focused on the Drosophila germline, where a signal mediated by the BMP type TGF-β growth factor Dpp emanates form a population of somatic niche stromal cells termed hub and contributes to the niche microenvironment that maintains the germline stem cell (GSC) pool. In the male this niche signal is tightly limited to those germline cells in direct contact with the Dpp source. Since in other contexts Dpp can signal over many cell diameters we speculated that a synapse-like tissue organization may constrain BMP spreading at the niche-GSC interface. We had at the start of the project developed GFP-based live reporters for BMP receptor activation that for the first time allowed detection of the active BMP receptor population live and at subcellular resolution. Consistent with the stem cell synapse hypothesis, we discovered in the course the project that BMP signalling in the fly testis niche is confined to small patches where the GSCs contact the BMP secreting niche cells. We next used fly genetics and cell biological methods to functionally dissect this potential signalling synapse. We could show that i) BMP signal transduction in the testis is restricted to adherens junctions between niche and stem cells, that ii) local signal transduction depends on exocyst mediated local release of BMP growth factors at these junctions, and iii) that similar mechanisms of secretion also seem to operate in situations when the same signalling molecules form long range gradients. While validating our approach we also generated a corresponding reporter for vertebrate BMP receptors. This was patented by the TUD as a tool for highly specific, fluorescence based high throughput screens for modulators of TGF-β pathway activation. We next followed up on observations we made on the expression of components of the Hedgehog (Hh) signalling pathway in the testis. Hh is, like Dpp and the cytokine like ligand Unpaired, produced by the hub cells. We could show that Hh is indeed another niche signal, albeit for the somatic stem cells associated with the GSCs which produce the cells that ensheath the differentiating germline clusters. This may reflect a deep conservation of niche organization between flies and mammals.
Projektbezogene Publikationen (Auswahl)
- Europäisches Patent EP 2141179 A1 und US patent 12/459,51, “A fluorescence based reporter construct for the direct detection of TGF-β receptor activation and modulators thereof”
Christian Bökel, Thomas Weidemann
- (2011) 'Local BMP receptor activation at adherens junctions in the Drosophila germline stem cell niche', Nat Commun 2: 415
Michel, M., Raabe, I., Kupinski, A. P., Perez-Palencia, R. and Bökel, C.
- (2012) 'Hh signalling is essential for somatic stem cell maintenance in the Drosophila testis niche', Development 139, 2663-69
Michel, M., Kupinski, A. P. and Bökel, C.
(Siehe online unter https://doi.org/10.1242/dev.075242)
