Kieran Read had been linked with Racing 92 and a Premiership switch, but said it was a "pretty straightforward" decision to move to Japan.
Toulouse's lead in the Top 14 table is down to a solitary point after Clermont Auvergne thrashed Grenoble. Lyon, meanwhile, lost at home.
Lyon claimed a third successive win and are breathing down Clermont's necks after beating them at Matmut Stade de Gerland,
Eddie Jones says only Manu Tuilagi can decide his future amid reports he is set for a big-money move to the Top 14.
After agreeing to join Racing 92 until the end of the season, Dan Carter expressed his sadness at the circumstances surrounding his return.
Racing 92 ran out of time in their defeat to Toulouse, while La Rochelle survived a Top14 scare against lowly Grenoble.
Racing 92 have some pretty formidable back-up for Finn Russell with Dan Carter returning to the Top14 club.
Clermont had not been beaten since October, but they were brought back down to earth by Top 14 champions Castres.
A spectator who abused Simon Zebo during Ulster's Champions Cup meeting with Racing 92 has been given a lifetime ban by the Pro14 club.
Saracens and Racing 92 secured home quarter-finals in the Champions Cup but the hopes of Exeter Chiefs are over after defeat at Munster.