Early embryogenesis requires tightly controlled temporal and spatial coordination of cellular behavior and signaling. Modulations are achieved at multiple levels, from cellular transcription to tissue-scale behavior. Intracellularly, the endolysosomal system emerges as an important regulator at different levels – but in vivo studies are rare. In the frog Xenopus, little is known about developmental roles of endosomal regulators, or their potential involvement in signaling, especially true for late endosomes. Here, we chose to analyze a hypothesized role of Rab7 in this context, a small GTPase known for its role as a late endosomal regulator. First, rab7 showed strong maternal expression. Following localized zygotic transcript enrichment in the mesodermal ring and neural plate, it was found in tailbud stage neural ectoderm, notochord, pronephros, eyes and neural crest tissues. Inhibition resulted in strong axis defects caused by a requirement of rab7 for mesodermal patterning and correct gastrulation movements. To test a potential involvement in growth factor signaling, we analyzed early Wnt-dependent processes in the mesoderm. Our results suggest a selective requirement for ligand-induced Wnt activation, implicating a context-dependent role of Rab7.

Early embryogenesis requires tightly controlled temporal and spatial activity of growth factors. Modulation is achieved at multiple levels, from cellular transcription to tissue-scale propagation, concerning activation, maintenance, or termination of signaling. Intracellularly, the endolysosomal system emerged as important factor for such regulations. Endocytosis and early endosomes mainly orchestrate activation and transduction of signals, while late endosomes are thought to mediate lysosome-based termination. Only few reports attributed activating roles to late endosomes, mainly for Wnt pathway activation. In Xenopus, few is known about developmental roles of endosomal regulators, or their function for signaling, especially true for late endosomes. Therefore, we chose to analyze a hypothesized positive role of Rab7 in this context, a small GTPase mainly known for its role as a late endosomal regulator. We analyzed if Rab7 participates in early activation of growth factor signaling, focusing on Wnt signaling. We found rab7 expressed very localized throughout development, and inhibition resulted in strong gastrulation defects. Concomitantly, Rab7 was strictly required for both, exogenous activation and endogenous Wnt-mediated patterning of the mesoderm.