Complex sentence: This was the first using ChR2 to steer the behavior of an animal in an optogenetic experiment, rendering a genetically specified cell type subject to optical remote control.
Split into simpler sentences: This was the first study using the remote control using ChR2 to steer the behaviour of an animal. It was the first using ChR2 for an optogenetic experiment-i.e. that rendered genetically specified cell types subject to optical remote control.