Drosophila's contribution to stem cell research

Introduction

The fundamental property of stem cells that they can not only differentiate into various types of cells, but can also renew the stem cell population, is the basis of progressive regenerative medicine¹. It is normal for some tissues like blood, skin, gut and germ cells to be regularly maintained by stem cell precursors. Stem cell niches control important properties of stem cells including self-renewing potential². Currently, Drosophila germ cells are established as a crucial model of stem cells.

Analysis of the recent literature

Drosophila ovary contains both germline and somatic stem cells that reside within the anterior region of each ovariole³. In an interesting experiment, where individual germaria, free of developing eggs and sheath tissue, were transplanted into the abdominal cavity of a host Drosophila, they not only regenerated ovariole-like structures but also maintained oogenesis⁴. Drosophila ovariole usually contains two somatic stem cells (called cystocysts) near the wall of the germarium. Interestingly, somatic stem cells, in this case, not only divide independently of surrounding cells, but also continue to divide in the absence of germline cells⁵. As asymmetric stem cell division is an important property that enables stem cells to self-renew and differentiate, the balance between symmetry and asymmetry is a tool that enables stem cells to maintain required numbers of progeny cells. An enormous amount of research effort has been directed towards understanding the basis of this asymmetry. Ablation of presumptive germline stem cells (GSCs) near the apical tip blocked the production of new germline cysts, however, previously initiated cysts were able to complete development in this case⁶. This indicated that development of cysts does not require continued cyst production. More importantly, ablation of a distinct group of somatic cells around GSCs leads to higher egg production⁷. It has been reported that stem cells adjust their proliferation rate in response to nutrition without changing the number of active stem cells e.g. a protein-rich diet increases the rate of egg production, in this case⁸.

Germline and somatic stem cells attach to form a cluster of cells (the hub) in the Drosophila testes. The hub expresses a ligand that activates the JAK-STAT signaling cascade⁹. Without this signal, GSCs do not self-renew, but can differentiate. Drosophila bag of marbles (bam) gene is required for the differentiation of daughter cells (cystoblasts) from mother stem cells¹⁰. Instead of differentiation, bam mutant germ cells proliferated like stem cells. Heat-induced bam expression caused elimination of female germinal stem cells while somatic stem cell numbers were not changed¹¹. Interestingly, ectopic bam expression had no such consequences on male germline cells indicating bam’s potential to regulate oogenesis and spermatogenesis in different ways¹¹. Somatic cyst cells and hub cells express two bone morphogenetic protein (BMP) molecules: Gbb (Glass bottom boat) and Dpp (Decapentaplegic). The Dpp/BMP signal was found to be essential for GSC maintenance¹². In absence of BMP signaling, bam is upregulated that can cause GSCs to be lost. Mutations in Dpp or its receptor (saxophone) increases stem cell loss and inhibits stem cell division. On the other hand, overexpression of Dpp blocks GSC differentiation¹³. Interestingly, BMP signaling reduces bam expression in ovarian GSCs. Phosphorylated Mad (pMad) is a direct indicator of BMP signaling as C-terminal phosphorylation of Mad by BMP receptor directs Mad toward BMP signaling¹⁴. Somatic inner germarium sheath cells failed to divide after removing GSC niches. Hedgehog (Hh) family signaling mediators are known for their important role during Drosophila development¹⁵. Hedgehog genes were also reported to be crucial for the proliferation of ovarian somatic cells in Drosophila. Drosophila neuroblasts regulate stem cell growth by separating the growth inhibitor Brat and the transcription factor Prospero into different daughter cells¹⁵. Interestingly, mutant Brat or Prospero caused both daughter cells to grow resulting into tumorigenesis¹⁶. High levels of Pumilio and Nanos proteins have also been observed in Drosophila GSCs¹⁷. Lack of zygotic activity of Nanos or Pumilio was found to have a dramatic effect on germline development in female flies. Pumilio mutant Drosophila not only failed to maintain stem cells but germline cells also¹⁷. Loqs protein was also found to be necessary for embryo survival and GSC sustenance in Drosophila. Decrease in stem cell functions could lead to the aging-related decline in tissue maintenance^18,19. Somatic niche cells in testes from aging males show reduced DE-cadherin and unpaired (Upd) proteins²⁰. Inside the Drosophila testes, Upd production in hub cells controls stem cell number within the niches, and overexpression of upd within niche cells can rescue GSCs even in case of aged males.

The identification of stem cell lineages in the midgut of Drosophila is a recent discovery^21–23. A genome-wide transgenic RNAi screen identified 405 genes that regulate intestinal stem cell (ISC) maintenance and differentiation in Drosophila intestine²⁴. By integrating these genes into functional networks, it was concluded that factors related to basic stem cell processes are commonly needed in all stem cells, and stem-cell-specific, niche-related signals are required only in the unique stem cell types. Analysis of genetic mosaics revealed that differentiated cells in the midgut epithelium come from a common lineage in Drosophila²⁵. Notch signaling controls key events during development. Consistent with its role of regulation of various adult stem cells, diminished notch signaling has been reported to cause increase in the number of precursor cells in the midgut of Drosophila²⁶.

Conclusions

For more than a century, Drosophila’s contribution to genetics and developmental biology has been enormous. With its increasing contribution to stem cell research, Drosophila consistently proves to be an invaluable model organism. Compared to mammalian model organisms, it is easy and inexpensive to work with Drosophila. Furthermore, shorter generation time, small size, and fewer ethical issues makes Drosophila an attractive animal model. Drosophila germline and midgut stem cells are currently being established as important models of stem cell research. Self-renewal of Drosophila GSCs requires both intracellular as well as extracellular signals. Several factors including BMP signals were found to be indispensable for sustaining GSCs in Drosophila. Asymmetric division of GSCs to produce and maintain a daughter GSC is regulated by gene expression in adjacent somatic cells also. In Drosophila, significant changes occur within the stem cell niche that contributes to a decline in stem cell number over time. These stem cell-related discoveries that were made in Drosophila, will surely be helpful for mammalian regenerative medicine, and more work is desperately needed in this area.