isn't there proof somewhere about Nikolai keeping Takeo as a prisoner when they were found by Richtofen or whoever.. and Dempsey was with a marine squad trying to save Peter and got caught up with those two(Tak & Nik) and the three were with Edward and he tried doing experiments on them that lead to their memory being erased from the 115?
The only place I've seen say that Takeo was a prisoner of Nikolai was a Theory that Alpha Snake put up about the whole zombies story line.
As there are a lot of blank patches in what we know, Alpha tried to fill in some of those blanks... it makes a good read. Other than that I've never seen it confirmed that Takeo was a prisoner of Nikolai.
I have however read in a number of places, that Dempsey, Nikolai and Takeo were prisoners of the Nazis and were experimented on with Element 115, as Red Russian says.
*EDIT* here is a link to Alpha's zombie storyline http://callofdutyzombies.com/forum/viewtopic.php?f=53&p=72877#p72877