Stage-specific differences in the requirements for germline stem cell maintenance in the Drosophila ovary

Dicer-1-dependent Dacapo suppression acts downstream of Insulin receptor in regulating cell division of Drosophila germline stem cells

It is important to understand the regulation of stem cell division because defects in this process can cause altered tissue homeostasis or cancer. The cyclin-dependent kinase inhibitor Dacapo (Dap), a p21/p27 homolog, acts downstream of the microRNA (miRNA) pathway to regulate the cell cycle in Drosophila melanogaster germline stem cells (GSCs). Tissue-extrinsic signals, including insulin, also regulate cell division of GSCs. We report that intrinsic and extrinsic regulators intersect in GSC division control; the Insulin receptor (InR) pathway regulates Dap levels through miRNAs, thereby controlling GSC division. Using GFP-dap 3'UTR sensors in vivo, we show that in GSCs the dap 3'UTR is responsive to Dicer-1, an RNA endonuclease III required for miRNA processing. Furthermore, the dap 3'UTR can be directly targeted by miR-7, miR-278 and miR-309 in luciferase assays. Consistent with this, miR-278 and miR-7 mutant GSCs are partially defective in GSC division and show abnormal cell cycle marker expression, respectively. These data suggest that the GSC cell cycle is regulated via the dap 3'UTR by multiple miRNAs. Furthermore, the GFP-dap 3'UTR sensors respond to InR but not to TGF-beta signaling, suggesting that InR signaling utilizes Dap for GSC cell cycle regulation. We further demonstrate that the miRNA-based Dap regulation may act downstream of InR signaling; Dcr-1 and Dap are required for nutrition-dependent cell cycle regulation in GSCs and reduction of dap partially rescues the cell cycle defect of InR-deficient GSCs. These data suggest that miRNA- and Dap-based cell cycle regulation in GSCs can be controlled by InR signaling.

miRNAs are essential for survival and differentiation of newborn neurons but not for expansion of neural progenitors during early neurogenesis in the mouse embryonic neocortex

Neurogenesis during the development of the mammalian cerebral cortex involves a switch of neural stem and progenitor cells from proliferation to differentiation. To explore the possible role of microRNAs (miRNAs) in this process, we conditionally ablated Dicer in the developing mouse neocortex using Emx1-Cre, which is specifically expressed in the dorsal telencephalon as early as embryonic day (E) 9.5. Dicer ablation in neuroepithelial cells, which are the primary neural stem and progenitor cells, and in the neurons derived from them, was evident from E10.5 onwards, as ascertained by the depletion of the normally abundant miRNAs miR-9 and miR-124. Dicer ablation resulted in massive hypotrophy of the postnatal cortex and death of the mice shortly after weaning. Analysis of the cytoarchitecture of the Dicer-ablated cortex revealed a marked reduction in radial thickness starting at E13.5, and defective cortical layering postnatally. Whereas the former was due to neuronal apoptosis starting at E12.5, which was the earliest detectable phenotype, the latter reflected dramatic impairment of neuronal differentiation. Remarkably, the primary target cells of Dicer ablation, the neuroepithelial cells, and the neurogenic progenitors derived from them, were unaffected by miRNA depletion with regard to cell cycle progression, cell division, differentiation and viability during the early stage of neurogenesis, and only underwent apoptosis starting at E14.5. Our results support the emerging concept that progenitors are less dependent on miRNAs than their differentiated progeny, and raise interesting perspectives as to the expansion of somatic stem cells.

Proliferative control in Drosophila stem cells

The relationship between cell growth (cell mass increase over time) and cell division is poorly understood in animal stem cells. Recent studies in several Drosophila stem cell types have provided the tools to interrogate this relationship. In several cases (brat, mei-P26, pros, bam, lethal giant larvae, polo), mutations have been defined that trigger tumorous overproliferation of progenitor cells and reveal how unrestricted self-renewing capacity is controlled. Moreover, microRNAs have been discovered as essential regulators of stem cell division rate and identity, suggesting that stem cell self-renewal depends on protein translational control. Biosynthetic capacity has also been found to be limiting for stem cell division rates. Finally, asymmetric cell division can impose dominant differentiation signals in a stem cell's daughter, and this can inhibit the stem cell-specific proliferation signature and lock in cell cycle exit.

Small RNAs: keeping stem cells in line

Stem cells and RNA silencing have emerged as areas of intense interest for both basic and clinical research. Recently these fields have converged with reports implicating small regulatory RNAs in the maintenance and pluripotency of stem cells.