Regulation of adult stem cell lineages [electronic resource] : the role of adhesion and intercellular signaling
Keywords: Genetics

Adult stem cells generate short-lived but highly differentiated cells that maintain and repair tissues which exhibit high turnover rates or tissues prone to wear and tear including sperm, skin, blood, intestinal epithelium and muscle. Adult stem cells typically reside in specialized microenvironments or stem cell niches, and a major goal of stem cell biology is to understand how these specialized microenvironments regulate adult stem cell self-renewal, proliferation and differentiation to maintain tissue homeostasis. Drosophila spermatogenesis provides an in vivo model system to investigate how the niche governs adult stem cell behavior. At the apical tip of the Drosophila testis, a cluster of post-mitotic support cells called the hub maintains two adult stem cell populations, germline stem cells (GSCs) and somatic cyst stem cells (CySCs). Hub cells secrete the Unpaired ligand, which activates JAK-STAT signaling in the neighboring GSCs and CySCs. JAK-STAT signaling and coordinated crosstalk between the germ line and soma together maintain GSCs and CySCs within the stem cell niche and direct proper association between their differentiating daughter cells. In this dissertation, I address two questions: (1) how a putative gene target of JAK-STAT influences stem cell self-renewal and maintenance and (2) how a transcriptional co-activator controls stem cell differentiation. To investigate the mechanism by which JAK-STAT signaling regulates GSC maintenance, I probed the function of profilin, a regulator of filamentous actin (F-actin). Profilin had been identified as a STAT target gene in the testis stem cell niche by performing chromatin immunoprecipitation with antibodies against phosphorylated STAT (activated STAT), followed by high throughput sequencing (ChIP-Seq) (Davies et al., submitted). It was already known that profilin mutants lose early germ cells (Gönczy and DiNardo, 1996) but the germline stem cell phenotype had not been characterized. I showed that GSCs were lost throughout development with markedly reduced GSC numbers in testes from late third instar larval hypomorphs and strong loss-of-function profilin mutants. By performing clonal analysis with null profilin alleles, I found that wild-type function of profilin was required cell autonomously for GSC maintenance. GSCs homozygous mutant for profilin detached from the hub and differentiated. F-actin and Adenomatous Polyposis Coli 2 (APC2) localization at the hub-GSC interface was disrupted in testes from profilin mutants. Interestingly, germline specific over-expression of APC2 rescued GSC loss in profilin hypomorphs. My studies suggested that profilin may be indirectly required to stabilize APC2 and [beta]-catenin at E-cadherin-based adherens junctions between hub cells and GSCs. Intercellular conversations between the germ line and the soma regulate the behavior of the stem cells in both lineages. I showed that a mutation in the MED20 gene, which encodes one of the subunits of Mediator, a multiprotein complex that acts as an interface between gene-specific regulatory proteins and the general transcriptional machinery to stimulate transcription, resulted in over-proliferation of both germline stem cells (GSCs) and somatic cyst stem cells (CySCs). Mediator function was required cell autonomously for CySCs to differentiate into somatic cyst cells, and non-autonomously for germ cell differentiation. This study reinforced the importance of crosstalk between stem cell lineages that work together to maintain tissue homeostasis.