Rbf Regulates Drosophila Spermatogenesis via Control of Somatic Stem and Progenitor Cell Fate in the Larval Testis

Abstract

SummaryThe Drosophila testis has been fundamental to understanding how stem cells interact with their endogenous microenvironment, or niche, to control organ growth in vivo. Here, we report the identification of two independent alleles for the highly conserved tumor suppressor gene, Retinoblastoma-family protein (Rbf), in a screen for testis phenotypes in X chromosome third-instar lethal alleles. Rbf mutant alleles exhibit overproliferation of spermatogonial cells, which is phenocopied by the molecularly characterized Rbf11 null allele. We demonstrate that Rbf promotes cell-cycle exit and differentiation of the somatic and germline stem cells of the testes. Intriguingly, depletion of Rbf specifically in the germline does not disrupt stem cell differentiation, rather Rbf loss of function in the somatic lineage drives overproliferation and differentiation defects in both lineages. Together our observations suggest that Rbf in the somatic lineage controls germline stem cell renewal and differentiation non-autonomously via essential roles in the microenvironment of the germline lineage.

A tumor suppressor Retinoblastoma1 is essential for embryonic development in the sea urchin

“…Thus, the cytoplasmic Rb1 may be simply stored inactive and/or may have distinct functions from its roles in the nucleus in these embryos. A similar cytoplasmic Rb1 expression has been also reported in Drosophila embryo, suggesting that cytoplasmic localization of Rb1 may be conserved among embryos of various organisms. Testing its possible cytoplasmic function during embryogenesis might be important in the future.…”

“…Although this mechanism may not be applicable in the germline, in the somatic lineage, this antagonistic relationship of Rb1 and Vasa appears to be critical for controlling a fine balance and timing of cell proliferation and differentiation during early embryogenesis of the sea urchin. Indeed, in this study, Rb1‐kd resulted in excessive Vasa protein accumulation in the entire embryo, mimicking tumorigenic cell development reported in other organisms . Perhaps, during early embryonic cell specifications, Rb1 serves as pluripotency suppressor while the germline factors serve as pluripotency activators, and a failure of this fine balance causes developmental failure, or in some cases tumorigenesis.…”

“…To test if the Rb1‐kd drives overexpression of the germline factors as seen in other organisms, we analyzed the expression levels of Vasa and Nanos 2 , representative germline markers present in various organisms at Day 1 (23–26 hpf). In the sea urchin embryo, Vasa mRNA is maternally present and becomes gradually restricted into the germline after blastula stage, while Nanos2 mRNA expression is more transient and starts after 32‐cell stage in the germline and decreases by Day 2.…”

“…Inhibition of each of those germline factors (e.g., vasa , piwi , aubergine , or nanos ) halted tumor growth, suggesting that Rb1‐mediated expression of these germline factors had an essential function in the somatic tumor . During embryonic development, on the other hand, Rb1 appears to play a critical role in promoting cell differentiation in several organisms such as Caenorhabditis elegans and Arabidopsis and its knockout drives overexpression of germline factors and prevents somatic differentiation . Thus, Rb1 appears to play a critical role in pluripotency control through the regulation of germline factors in both cancer cells and embryonic cells.…”

“…10 During embryonic development, on the other hand, Rb1 appears to play a critical role in promoting cell differentiation in several organisms such as Caenorhabditis elegans and Arabidopsis and its knockout drives overexpression of germline factors and prevents somatic differentiation. 11,12 Thus, Rb1 appears to play a critical role in pluripotency control through the regulation of germline factors in both cancer cells and embryonic cells. Based on these observations, we hypothesize that a fine balance of Rb1 (pluripotency suppressor) and germline factors (pluripotency activators) is essential for proper pluripotency regulation in the cells, and that its failure in some cases leads to cancers.…”