Nogo-A-cleaved amino-terminal fragment but not Nogo-B regulates STAT3 activation

Nogo family protein contains splice variants Nogo-A and Nogo-B, which differ in their expression patterns and functions. Ectopic overexpression of Nogo-A in HEK293T cells causes multiple fragmentation of the Nogo-A protein. Herein, we focused on the posttranslational production of a Nogo-A amino-terminal fragment with a molecular weight of approximately 45 kDa, which we term NogoA-213. Comparing endogenous Nogo protein expression among mouse organs, we detected full-length Nogo-A protein migrating at approximately 200 kDa and a smaller protein of approximately 45 kDa using an anti-Nogo-A N-terminal antibody. Interestingly, this 45 kDa band was recognized by an anti-NogoA-F antibody that detects human Nogo-A aa 186-213, which is specific to full-length Nogo-A and the Nogo-A N-terminal fragment but not Nogo-B, and completely overlapped with the bands detected by anti-NogoA N-terminal antibody. Therefore, we concluded that the 45 kDa band represents the Nogo-A N-terminal fragment rather than Nogo-B. Nogo proteins contain a hydrophobic domain in their C-terminal region and localize to membrane organelles such as the endoplasmic reticulum (ER) and plasma membrane. Following overexpression, Nogo-B localized to the ER and plasma membrane in HeLa cells, whereas Nogo-213 diffused throughout the cell because of lacking the C-terminal hydrophobic domain. Furthermore, following overexpression, NogoA-213 colocalized with signal transducer and activator of transcription 3 (STAT3) and enhanced leukemia inhibitory factor (LIF)-induced STAT3 phosphorylation, whereas Nogo-B did not. Importantly, LIF treatment promoted the generation of the Nogo-A N-terminal fragment in HEK293T cells. Taken together the NogoA-N-terminal fragment NogoA-213 but not Nogo-B plays a role in STAT3 activation.