I recently had a discussion where I was told that in an $\ce{^1H}/\ce{^13C}$-HMBC experiment coupling through an $\ce{C-O-C}$ ether bond would not be observed with the reasoning that O is not NMR active. That somehow surprised me as I thought only the "end nodes" in the coupling needed to be NMR active. So I have the following two questions:
Do the nuclei in-between need to be NMR-active?
Does the end node X need to be NMR-active?