451C Poster - 05. Reproduction and gametogenesis
Saturday April 09, 1:30 PM - 3:30 PM

Authors:
Nicole Green; Tina Tootle

Affiliation: Carver College of Medicine, University of Iowa, Iowa City, IA

Keywords:
n. stem cells; b. germline stem cell

While actin was observed in the nucleus decades ago, nuclear functions of actin dynamics have only recently become widely acknowledged. Indeed, nuclear actin regulates the activity of RNA polymerases and transcription factors, chromatin organization via remodeling complexes and histone deacetylases, and nuclear integrity. Furthermore, nuclear actin is involved in determining differentiation state, deciding cell identity, and reprogramming cells to a pluripotent state. Our studies investigate the roles of nuclear actin in regulating stemness using a model tissue, the Drosophila ovary. The Drosophila ovary is made up of 15-20 ovarioles of sequentially developing follicles. Germline stem cells (GSCs) reside in a niche at the anterior tip of each ovariole in a structure known as the germarium and give rise to all germline cells. GSCs are maintained by asymmetric divisions that retain a stem cell and produce a daughter cell, which continues to incompletely divide to produce 2-, 4-, 8-, and 16-cell germline cysts. Our lab previously defined several distinct pools of nuclear actin in the ovary by screening established actin labeling tools. One of these actin pools stained by anti-actin C4 is found in both the nucleoplasm and nucleolus of GSCs. These C4 positive nuclear actin pools are present in a dynamic pattern across the germarium and are lost as cells become more differentiated. This trend suggests nuclear actin may play a role in regulating stemness. To test this hypothesis, we overexpressed NLS-actin constructs in germline cells of the germarium. When we increased monomeric nuclear actin (NLS-Act5C^G13R), GSC maintenance is disrupted and there is progressive germline loss which results in empty germaria. As nuclear actin is enriched in the nucleolus and previous studies uncovered that nucleolar functions are essential for GSC maintenance, we were curious whether nuclear actin regulates nucleolar functions. Using nucleolar structure as a readout for nucleolar function, we find that increasing clear monomeric actin results in increased nuclear size and hypertrophic nucleoli. These nucleoli are also deformed and fragmented suggesting altered nucleolar functions. These results indicate that nuclear actin dynamics regulate key nuclear functions, including nucleolar activity, which are necessary for maintaining stem cells.