Regulation of Toll and Toll-like receptor signaling by the endocytic pathway

The Toll/TLR receptor family plays a central role in both vertebrate and insect immunity, driving the activation of humoral immunity in response to pathogens. In Drosophila, Toll is also responsible for directing the formation of the Dorsal/NFkappaB gradient specifying dorsoventral patterning of the embryo. Two recent studies have revealed that endocytosis and elements of the molecular machinery governing endosomal progression are required for Drosophila Toll signaling in development and immunity. We demonstrated that Toll is not only present at the plasma membrane but also in a Rab5+ early endosomal compartment in the embryo and that the distribution of constitutively active Toll10B is shifted towards endosomes. Localized inhibition of Rab5 function on the ventral side leads to a reduction of nuclear Dorsal levels, while locally increasing Rab5 function leads to potentiation of signaling. Independently, another laboratory identified the endosomal protein Mop as a potentiator of Toll signaling in Drosophila cell culture and fat-body tissue. Mop functions together with the ESCRT 0 component, Hrs, previously reported to stimulate endosomal progression and the signaling ability of internalized EGFR. We discuss these studies and briefly summarize the most significant findings concerning the role of intracellular localization and trafficking in mammalian TLR function

Autoproteolysis and feedback in a protease cascade directing Drosophila dorsal–ventral cell fate

Three serine protease zymogens, Gastrulation defective (GD), Snake (Snk) and Easter (Ea), and a nerve growth factor-like growth factor ligand precursor, Spaetzle, are required for specification of dorsal– ventral cell fate during Drosophila embryogenesis. The proteases have been proposed to function in a sequential activation cascade within the extracellular compartment called the perivitelline space. We examined biochemical interactions between these four proteins using a heterologous co-expression system. The results indicate that the three proteases do function in a sequential activation cascade, that GD becomes active and initiates the cascade and that interaction between GD and Snk is sufficient for GD to cleave itself autoproteolytically. The proteolytically active form of Ea cleaves GD at a different position, revealing biochemical feedback in the pathway. Both GD and Snk bind to heparin–Sepharose, providing a link between the pipe-defined ventral prepattern and the protease cascade. Our results suggest a model of the cascade in which initiation is by relief from inhibition, and spatial regulation of activity is due to interaction with sulfated proteoglycans