RESUMEN
Activation and repression of Notch target genes is mediated by transcription factor CSL, known as Suppressor of Hairless (Su(H)) in Drosophila and CBF1 or RBPJ in human. CSL associates either with co-activator Notch or with co-repressors such as Drosophila Hairless. The nuclear translocation of transcription factor CSL relies on co-factor association, both in mammals and in Drosophila. The Drosophila CSL orthologue Su(H) requires Hairless for repressor complex formation. Based on its role in transcriptional silencing, H protein would be expected to be strictly nuclear. However, H protein is also cytosolic, which may relate to its role in the stabilization and nuclear translocation of Su(H) protein. Here, we investigate the function of the predicted nuclear localization signals (NLS 1-3) and single nuclear export signal (NES) of co-repressor Hairless using GFP-fusion proteins, reporter assays and in vivo analyses using Hairless wild type and shuttling-defective Hairless mutants. We identify NLS3 and NES to be critical for Hairless function. In fact, HâNLS3 mutant flies match H null mutants, whereas HâNLS3âNES double mutants display weaker phenotypes in agreement with a crucial role for NES in H export. As expected for a transcriptional repressor, Notch target genes are deregulated in HâNLS3 mutant cells, demonstrating nuclear requirement for its activity. Importantly, we reveal that Su(H) protein strictly follows Hairless protein localization. Together, we propose that shuttling between the nucleo-cytoplasmic compartments provides the possibility to fine tune the regulation of Notch target gene expression by balancing of Su(H) protein availability for Notch activation.
Asunto(s)
Citoplasma/metabolismo , Proteínas de Drosophila/metabolismo , Drosophila melanogaster/genética , Drosophila melanogaster/metabolismo , Receptores Notch/metabolismo , Factores de Transcripción/metabolismo , Animales , Proteínas de Drosophila/genética , Femenino , Señales de Exportación Nuclear/genética , Señales de Localización Nuclear/genética , Fenotipo , Receptores Notch/genética , Factores de Transcripción/genética , Alas de Animales/crecimiento & desarrolloRESUMEN
Germline stem cell development and differentiation is tightly controlled by the surrounding somatic cells of the stem cell niche. In Drosophila females, cells of the niche emit various signals including Dpp and Wg to balance stem cell renewal and differentiation. Here, we show that the gene pzg is autonomously required in cells of the germline to sustain the interplay between niche and stem cells. Loss of pzg impairs stem cell differentiation and provokes the death of cells in the germarium. As a consequence of pzg loss, increased growth signalling activity predominantly of Dpp and Wg/Wnt, was observed, eventually disrupting the balance of germ cell self-renewal and differentiation. Whereas in the soma, apoptosis-induced compensatory growth is well established, the induction of self-renewal signals during oogenesis cannot compensate for dying germ cells, albeit inducing a new niche-like microenvironment. Instead, they impair the further development of germ cells and cause in addition a forward and feedback loop of cell death.