EDIT: On a really sad note, it's been announced on the official site that Nana is going to leave the group next week I'm going to miss her a lot...
It shouldn't come as a surprise that Ayano Nana left the group. She hasn't attended any TV show recording nor live shows since October, so it is pretty obvious what happened. The only reason they didn't announce it until now is because she is a part of their second trading card set, but now the promotion for the trading card is over, they can officially part ways.
However, there are far worst news than Ayano Nana leaving. Albeit, unconfirmed, it is almost certain that Amatsuka Moe, Rirei and one of the new member, Hasegawa Rui have all quit. Hasegawa Rui leaving isn't really a big deal since she hasn't done anything yet. Rirei is popular among female fans because of her looks, so it might affect a little. On the other hand, Amatsuka Moe leaving will be a major loss for Muscats. The people running Muscats should be blamed for not able to keep her on board since she is undoubtedly the best at attracting fans in the AV industry. It is crazy how far people are willing to go to get tickets to her events. To be honest, with her extreme popularity, the Muscats need her way more than she needs Muscats.
On the bright side of things, it is very possible that Sakura Yura will be making a comeback sometime in the near future.
update: Amatsuka Moe announced her graduation and Sakura Yura officially returned in this week's show. They also announced on the show that they will release a mini CD album on April 26.