Regulation of germline differentiation by the somatic lineage in Drosophila spermatogenesis [electronic resource]
Keywords: Developmental Biology.

Adult stem cells are undifferentiated cells that are capable of both self-renewal and differentiation into a variety of specialized cells. Adult stem cell lineages have been identified in several organs including skin, blood, breast, intestine, bladder, skeletal muscle, prostate, and the testes. The changes in homeostatic conditions in these organs caused by tissue turnover or injury rely heavily on the pool of adult stem cells for cell replenishment. Therefore, the activity of adult stem cells must be tightly controlled, and many of these regulations are orchestrated by the stem cell niche, the local microenvironment in which stem cells reside. The fast generation cycle and the genetic tractability of fruit flies make the Drosophila testis stem cell niche an ideal system for the study of adult stem cell regulation. The Drosophila testis maintains two types of stem cells: germline stem cells (GSCs) which give rise to sperm, and cyst stem cells (CySCs) which differentiate into cyst cells that encapsulate germ cells. The somatic cyst cell lineage has been implicated to be required at several stages of spermatogenesis and play pivotal roles in germline proliferation, survival, and differentiation. This dissertation focuses on understanding the role of the cyst cell lineage in GSC maintenance and investigating the mechanisms by which cyst cells direct early germ cell differentiation. Previously, it has been proposed that CySCs are the source of instructive self-renewal cues for GSCs. In contrast to this model, I showed that early germ cells with GSC characteristics can be maintained in the absence of CySCs and cyst cells. These germ cells failed to enter the transit-amplifying program, which is regarded as the first step of differentiation in many adult stem cell lineages. My observations suggest that cyst cells provide a pro-differentiation environment for GSCs, and that this mechanism(s) may be repressed in CySCs which indirectly allow for GSC self-renewal. Encapsulation of germ cells by cyst cells is one of the differentiation-promoting mechanisms imposed by the soma on the germline, and I have uncovered that this process requires activation of the EGFR-Ras-MEK-MAPK pathway in differentiated cyst cells. Furthermore, I also showed that repression of EGFR activation in CySCs is important in maintaining the population of GSCs and CySCs at the niche, as premature activation of the pathway resulted in displacement of GSCs from the hub by somatic cells. Preliminary studies revealed a STAT target gene, Socs36E as a negative regulator of the EGFR pathway in CySCs to prevent out-competition of GSCs by somatic cells. To understand how the somatic cyst cells germline differentiation, I performed two genetic screens: an RNAi screen of genes that caused premature germ cell differentiation when misexpressed in the soma, and a misexpression screen of genes predicted to be cell-surface and secreted proteins. Three promising candidates were identified through these screens: two ribosomal subunits, Rpl13A and RpS10a; and a septate junction component, Neurexin IV. The two ribosomal proteins may be involved in the soma to promote germ cell encapsulation. Neurexin IV however, operates non-cell autonomously in cyst cells to regulate germ cell differentiation into spermatocytes. Together, the results of this dissertation emphasize the importance and complexity of the interaction between adult stem cells and their microenvironment in maintaining tissue homeostasis.