SCFFBXO21-mediated ubiquitination and degradation of NMNAT2 regulates axon survival in nerve injury.

NMNAT2 is an essential but labile protein required for axon integrity. It is rapidly degraded after nerve injury, promoting axon degeneration. However, the mechanisms regulating NMNAT2 ubiquitination and turnover in neurons remain unclear. In this study, we identify the F-box protein FBXO21 as an NMNAT2-binding protein, and its deficiency confers axonal protection via increasing NMNAT2 abundance. FBXO21 recruits SKP1, CUL1, and RBX1 to form an SCFFBXO21 complex, which promotes NMNAT2 ubiquitination in vivo and in vitro. SCFFBXO21 ubiquitinates NMNAT2 at K155 within an isoform-specific targeting and interaction domain of the family of NMNATs, which underlies the unique labile nature of NMNAT2. The ubiquitination-deficient NMNAT2-K155R exhibits substantially reduced protein turnover and enhanced axon-protective capacity. Finally, in Fbxo21 knockout mice, NMNAT2 levels are markedly increased and the survival of injured sciatic nerves is significantly prolonged. Collectively, our findings reveal a crucial role of FBXO21 in axon degeneration, highlighting the SCFFBXO21 complex as a potential target for modulating NMNAT2-dependent axon survival.