Importin α (Impα) proteins are required for transporting proteins from the cytoplasm into the nucleus via interaction with Impβ in all tissues but also have roles in transcriptional regulation and organisation of chromatin. The Drosophila melanogaster genome encodes four Impα genes (1-4). We have identified a specific requirement for Impα1 in maintenance of male germline stem cells and spermatogonial differentiation. Germline stem cells (GSCs) are restricted to the apical testis where the stem cell niche responsible for regeneration is located. 8-10 GSCs surround a group of somatic niche cells termed the hub. Mitotic division of a GSC results in a daughter remaining attached to the hub that maintains GSC identity and a daughter displaced from the hub that becomes a gonialblast, or primary spermatogonial cell. The gonialblast undergoes four more rounds of mitosis characterised by incomplete cytokinesis to produce a cyst of 16 interconnected spermatogonia which then proceed through pre-meiotic S-phase and differentiate as spermatocytes.
Loss of function Impα1 mutants lose GSCs and this loss can be rescued by germline specific expression of Impα but the rescue animals are still infertile indicating a secondary role of Impα in spermatogenesis. The Impα mutant phenotype can be phenocopied by germline expression of a dominant-negative Impα protein that lacks the Impβ binding domain. This phenotype can be characterized by a loss of early germline cells and an increase in the size of the hub. We are currently screening molecules that may be transported by Impα into germ cell nuclei to regulate GSC maintenance and differentiation.