MicroRNA-190 downregulates Bag of marbles to allow the switch from proliferation to differentiation in the Drosophila male germline stem cell lineage. Gonzalo H Olivares, Margaret T Fuller. Developmental Biology, Stanford University School of Medicine, Palo Alto, CA.

   In many adult stem cell lineages, stem cell daughters commonly undergo a limited number of transit amplifying (TA) mitotic divisions before initiating terminal differentiation, allowing production of many differentiated progeny per stem cell division. The number of TA divisions must be tightly regulated: too few may lead to defective tissue regeneration, too many to abnormal growth and cancer. In the Drosophila male germline stem cell lineage, Bag of marbles (Bam) is required for cessation of TA cell divisions and onset of spermatocyte differentiation. Loss of function of bam causes male TA germ cells to continue proliferation without initiating differentiation. We have demonstrated that Mei-P26 facilitates accumulation of Bam protein in TA cells to allow the switch from proliferation to differentiation. Mei-P26 physically interacts with Ago1 and represses expression of micro-RNAs (miRNAs), suggesting that Mei-P26 regulates Bam protein expression post-transcriptionally via its 3'UTR. To test this hypothesis, I screened for miRNAs that when over-expressed generate a phenotype that resembles a bam mutant. I found that ectopic expression of miR-190 in germ cells caused overproliferation of spermatogonia resembling loss of function of bam, with germ cells enclosed in cysts and having branched fusomes consistent with TA identity. As in Mei-P26 mutants, miR-190 over-expression (OE) shows low levels of Bam protein, suggesting that forced miR-190 OE prevented Bam protein levels from reaching the threshold required to stop proliferation. A bam 3UTR reporter with mutated miR-190 binding site show no repression at late stages. I am currently testing the model that the tumor suppressor Mei-P26 regulates expression or action of miR-190 to allow accumulation of Bam in early germ cells.