Mei-P26 regulates microRNAs and cell growth in the Drosophila ovarian stem cell lineage

Abstract

Drosophila neuroblasts 1 and ovarian stem cells 2 , 3 are well characterized models for stem cell biology. In both cell types, one daughter cell self-renews continuously while the other undergoes a limited number of divisions, stops to proliferate mitotically and differentiates. Whereas neuroblasts segregate the Trim-NHL (tripartite motif and Ncl-1, HT2A and Lin-41 domain)-containing protein Brain tumour (Brat) into one of the two daughter cells 4 -6 , ovarian stem cells are regulated by an extracellular signal from the surrounding stem cell niche. After division, one daughter cell looses niche contact. It undergoes 4 transit-amplifying divisions to form a cyst of 16 interconnected cells that reduce their rate of growth and stop to proliferate mitotically. Here we show that the Trim-NHL protein 8 ) restricts growth and proliferation in the ovarian stem cell lineage. Mei-P26 expression is low in stem cells but is strongly induced in 16-cell cysts. In mei-P26 mutants, transit-amplifying cells are larger and proliferate indefinitely leading to the formation of an ovarian tumour. Like brat, mei-P26 regulates nucleolar size and can induce differentiation in Drosophila neuroblasts, suggesting that these genes act through the same pathway. We identify Argonaute-1, a component of the RISC complex, as a common binding partner of Brat and Mei-P26, and show that Mei-P26 acts by inhibiting the microRNA pathway. Mei-P26 and Brat have a similar domain composition that is also found in other tumour suppressors and might be a defining property of a new family of microRNA regulators that act specifically in stem cell lineages.When Drosophila germline stem cells divide (Fig. 1a) Fig. 3h and Supplementary Fig. 4a, b). A.F. contributed the S2 luciferase assay ( Supplementary Fig. 7c, d). I.P. assisted in the experiment in Fig. 3f, g and performed experiments in the larval brain ( Supplementary Fig. 3). S.C. and N.B. designed the microRNA quantitative PCR experiment, which was performed by N.B. (Fig. 4a and Supplementary Fig. 7a, b). J.A.K. wrote the paper. Author Information Reprints and permissions information is available at www.nature.com/reprints. Correspondence and requests for materials should be addressed to J.A.K. (juergen.knoblich@imba.oeaw.ac.at).Full Methods and any associated references are available in the online version of the paper at www.nature.com/nature.Supplementary Information is linked to the online version of the paper at www.nature.com/nature. (Fig. 1b-f). Wild-type ovaries contain two stem cells near the tip of the germarium whereas cystoblasts and cysts are separated from the stem cell niche (Fig. 1b). In mei-P26 mutant ovaries, germaria are filled with individual spectrosome-containing cells (19%) and cysts containing varying numbers of cells (<4 in 39%, >4 in 42%; n > 400 cells) connected by a fusome (Fig. 1c, d and Supplementary Fig. 1a, a'). Nurse cells and oocytes are not formed, and fusomes or spectrosomes are maintained at later stages of oogenesis (Fig. 1e, f). In wild-type ovaries, the ...

Vasa promotes Drosophila germline stem cell differentiation by activating mei-P26 translation by directly interacting with a (U)-rich motif in its 3′ UTR

“…As cells with punctate spectrosomes as well as cells with branched fusomes were observed, we conclude that the vas 1 /vas PH165 ; mei-P26 1 /mei-P26 fs1 egg chambers contain mostly early-stage cystocytes, a phenotype also characteristic of strong mei-P26 alleles (Fig. 2G,H;Neumü ller et al 2008). Thus reduction of vas activity enhances the phenotypic consequences of a hypomorphic mei-P26 allelic combination.…”

“…vas and mei-P26 interact genetically Among the mRNAs we recovered was mei-P26, which has been implicated in germ cell differentiation in both sexes, and which also functions somatically in the nervous system (Page et al 2000;Glasscock et al 2005;Neumü ller et al 2008). Mei-P26, a TRIM-NHL domain protein, interacts with Argonaute-1 (AGO1), inhibiting the microRNA (miRNA) pathway and promoting differentiation of early-stage committed germline cells (Neumü ller et al 2008).…”

“…Mei-P26, a TRIM-NHL domain protein, interacts with Argonaute-1 (AGO1), inhibiting the microRNA (miRNA) pathway and promoting differentiation of early-stage committed germline cells (Neumü ller et al 2008). To test for a common function, we investigated whether vas and mei-P26 mutations interact.…”

“…4A,B;Neumü ller et al 2008). We used mei-P2 6fs1 /mei-P26 mfs1 ovaries as negative controls and to ensure the specificity of the immunostaining reaction (Fig.…”

Attributes of adult stem cells

“…In Drosophila neuroblasts, a conserved polarity complex (the Par complex) of atypical protein kinase C (aPKC), Par6 and Bazooka (Par3 in other species) serves to orientate the mitotic spindle along the basal-apical axis [18, in this issue], or cell fate determinants are asymmetrically partitioned to daughter cells that are destined to differentiate ( Figure 3DII). Examples include the distribution of inhibitors of self-renewal, such as Brat and Mei-P26, to daughter cells destined to become TACs in the Drosophila CNS and ovaries, respectively [19,20], and the similar destination of Numb, an inhibitor of Notch signalling, that has a conserved role of controlling lineage specification in a variety of cell types [21]. The relationship between these two intrinsic mechanisms is becoming clearer [22]; in the Drosophila CNS, Aurora A phosphorylates Par6, a regulatory subunit of aPKC that becomes activated and phosphorylates Lethal (2) giant larvae (Lgl), an interphase inhibitor of aPKC.…”