Vitellogenesis in Oncopeltus fasciatus: PLC/IP3, DAG/PK-C pathway triggered by CaM

In Oncopeltus fasciatus, evidence shown here indicates it is calmodulin (CaM) that activates phospholipase-C (PLC), beginning a signalling pathway necessary for endocytic uptake of yolk precursor molecules. Epithelial cell-produced CaM, transported to oocytes via gap junctions, has been shown to be required for receptor-mediated endocytic uptake of vitellogenins (Vgs, the protein precursors of yolk). To determine if CaM was directly or indirectly stimulating the phospholipase-C (PLC) signalling cascade and thus controlling Vg endocytosis we used a series of molecules known to inactivate various elements of the pathway. W-7 prevents CaM from interacting with other molecules. Neomycin isolates PIP2 from PLC. U-73122 directly inactivates PLC. 2-APB blocks IP3 receptors which would otherwise cause release of Ca2+. Verapamil and CdCl2 block Ca2+ release channels. Staurosporin and calphostin are inhibitors of PK-C. 1-Hexadecyl-2-acetyl glycerol (HAG) binds to diacylglycerol (DAG). Through the use of these antagonists we show here that: (1) the activation of phospholipase-C in this system requires CaM. (2) Stimulated phospholipase-C converts PIP2 into IP3 and DAG. (3) IP3 causes increase in cytosolic Ca2+. (4) DAG and Ca2+ each stimulate phosphokinase-C, resulting in endocytosis of Vgs.