An RNA-binding regulatory cascade controls the switch from proliferation to differentiation in the <i>Drosophila</i> male germ cell lineage.

Abstract

The switch from precursor cell proliferation to onset of differentiation in adult stem cell lineages must be carefully regulated to produce sufficient progeny to maintain and repair tissues, yet prevent overproliferation that may enable oncogenesis. In the <i>Drosophila</i> male germ cell lineage, spermatogonia produced by germ line stem cells undergo a limited number of transit amplifying mitotic divisions before switching to the spermatocyte program that sets up meiosis and eventual spermatid differentiation. The number of transit amplifying divisions is set by accumulation of the <i>bag-of-marbles</i> (Bam) protein to a critical threshold. In <i>bam</i> mutants, spermatogonia proliferate through several extra rounds of mitosis and then die without becoming spermatocytes. Here, we show that a key role of Bam for the mitosis to differentiation switch is repressing expression of Held Out Wings (<i>how</i>), homolog of mammalian Quaking. Knockdown of <i>how</i> in germ cells was sufficient to allow spermatogonia mutant for <i>bam</i> or its partner <i>benign gonial cell neoplasm</i> to differentiate, while forced expression of nuclear-targeted How protein in spermatogonia wild-type for <i>bam</i> resulted in continued proliferation at the expense of differentiation. Our findings suggest that Bam targets <i>how</i> RNA for degradation by acting as an adapter to recruit the CCR4-NOT deadenylation complex via binding its subunit, Caf40. As How is itself an RNA-binding protein with roles in RNA processing, our findings reveal that the switch from proliferation to meiosis and differentiation in the <i>Drosophila</i> male germ line adult stem cell lineage is regulated by a cascade of RNA-binding proteins.