Investigating the role of Hh signaling during embryonic germ cell migration in Drosophila melanogaster

Abstract

In Drosophila melanogaster, the embryonic gonad develops from primordial germ cells (PGCs), which are specified early in development by maternal determinants, and somatic gonadal precursor cells (SGPs), which arise from the mesoderm by the action of zygotic patterning genes. As the PGCs and SGPs are specified at distant locations, primitive gonad formation requires directed cell migration of PGCs through the midgut and mesoderm. The precise trajectory of PGCs is governed by repulsive cues mediated by Wunens (lipid phosphate phosphatases) and by attractive cues, which this study presents are produced and interpreted by a non-canonical Hedgehog (Hh) signaling pathway. While canonical Hh signaling is a highly conserved pathway required for embryonic patterning and cell fate specification, non-canonical Hh pathways that function to guide migrating cells have been discovered in a variety of biological contexts, particularly in axon guidance within the vertebrate nervous system. Studies over the last two decades have identified that germ cell migration factors like Hmgcr specifically potentiate the transmission of the Hh ligand, and they have since connected reception of the ligand by PGCs with the downstream membrane localization of the GPCR, Trapped in endoderm-1 (Tre1), which remodels the actin cytoskeleton to promote directed migration. Further supporting these claims, this thesis demonstrates through analysis of migration phenotypes and Hh pathway gene expression that: i) Hh functions in the embryonic mesoderm to attract PGCs throughout migration, ii) Hmgcr is also required in the mesoderm to potentiate Hh chemoattraction and acts upstream of Hh and its regulators, and iii) proteins required for Hh signal reception are also required in germ cells for proper migration. Additionally, this study demonstrates the ability of Wunens to suppress Hh signaling, suggesting that Hmgcr and Wunen modulate Hh signaling in an opposing manner to define the path of migrating PGCs. Uncovering the mechanistic underpinnings of directed cell motility is invaluable to our understanding of many biological processes including cancer metastasis, cellular immunity, and fetal organogenesis.