Lin28 is a critical factor in the function and aging of Drosophila testis stem cell niche

Abstract

Age-related decline in stem cell function is observed in many tissues from invertebrates to humans. While cell intrinsic alterations impair stem cells, aging of the stem cell niche also significantly contributes to the loss of tissue homeostasis associated with reduced regenerative capacity. Hub cells, which constitute the stem cell niche in the Drosophila testis, exhibit age-associated decline in number and activities, yet underlying mechanisms are not fully understood. Here we show that Lin28, a highly conserved RNA binding protein, is expressed in hub cells and its expression dramatically declines in old testis. lin28 mutant testes exhibit hub cell loss and defective hub architecture, recapitulating the normal aging process. Importantly, maintained expression of Lin28 prolongs hub integrity and function in aged testes, suggesting that Lin28 decline is a driver of hub cell aging. Mechanistically, the level of unpaired ( upd) , a stem cell self-renewal factor, is reduced in lin28 mutant testis and Lin28 protein directly binds and stabilizes upd transcripts, in a let-7 independent manner. Altogether, our results suggest that Lin28 acts to protect upd transcripts in hub cells, and reduction of Lin28 in old testis leads to decreased upd levels, hub cell aging and loss of the stem cell niche.

Proliferative stem cells maintain quiescence of their niche by secreting the Activin inhibitor Follistatin

“…Finally, during normal aging, the ability of the hub to support stem cells declines. This is due both to a reduction in hub cell numbers ( Sreejith et al., 2019 ; Wallenfang et al., 2006 ; Lee et al., 2016 ), as well as lower production of self-renewal ligands to support surrounding stem cells ( Toledano et al., 2012 ; Boyle et al., 2007 ). Yet how the mechanisms maintaining hub cell quiescence are affected during aging is still unknown.…”

Lin28 and Imp are Required for Stability of Bowl Transcripts in Hub Cells of the Drosophila Testis

“…Lin28 was previously shown to be required for stabilizing transcripts of Upd, an important hub cell factor (Sreejith et al, 2019). This observation prompted us to identify other targets that are stabilized by Lin28 in hub cells of the Drosophila testis.…”

“…To further investigate the molecular mechanisms underlying promotion of Bowl transcript stability by Lin28 and Imp, we carried out cell culture experiments using Luciferase (Luc) constructs bearing the Bowl 3'UTR (Luc-Bowl 3'UTR), employing RT-PCR to examine whether those hybrid transcripts were stabilized by Lin28 and Imp, which stabilizes target mRNAs through 3'UTR (Toledano et al, 2012;Lee et al, 2019;Sreejith et al, 2019). When either Lin28 or Imp was co-transfected with the construct, Luc-Bowl 3'UTR transcript levels were increased (Fig.…”

“…Notably, the embryonic gonads of Bowl mutants contain fewer hub cells, while increased Bowl activity transforms CySCs to the hub cell fate (DiNardo et al, 2011); these findings indicate that Bowl is a hub cell specification factor. Similarly, Lin28 mutant embryonic gonads contain fewer hub cells (Sreejith et al, 2019), suggesting that Lin28 could function in hub cell specification together with Bowl. Here we find that in hub cells, Bowl transcripts require Lin28 and Imp for stability.…”

“…Within the Drosophila testis, the RNA binding protein Lin28 is exclusively expressed in hub cells (Sreejith et al, 2019). Lin28 not only acts as a repressor of Let-7 biogenesis but also controls the stability and translation of target mRNAs including Unpaired (Upd), an extrinsic stem cell selfrenewal factor that is expressed and secreted from hub cells (Kiger et al, 2001;Tulina & Matunis, 2001;Sreejith et al, 2019). A second RNA binding protein from a conserved protein family, IGF-II messenger RNA binding protein (Imp), is also required for stabilizing Upd transcripts (Toledano et al, 2012).…”