Mouse spermine oxidase gene splice variants - Nuclear subcellular localization of a novel active isoform

Spermine oxidase (SMO) is a flavoenzyme involved in polyamine homeostasis in animal cells. The mouse spermine oxidase gene (mSMO) codes for splice variants, including the previously reported major active isoform, herein named alfa (alpha). In the present work, eight additional gene splicing variants were characterized. The heterologous expression and biochemical characterization of three recombinant isoforms (namely mSMOmu, -gamma and -delta) revealed that only the recombinant protein mSMOmu displays biochemical characteristics similar to those of mSMOalpha; the other two recombinant proteins contained no detectable SMO activity. In order to investigate in greater detail, the SMO enzyme activity associated with their subcellular localization, mSMOalpha and -mu V5-tagged proteins were transiently and stably transfected in the murine neuroblastoma cell line, N18TG2. Very interestingly, the novel active mSMOmu isoform was found to be present in both nuclear and cytoplasmic compartments, thus providing the first evidence of SMO activity in the nucleus, while a cytoplasmic localization was confirmed for the mSMOalpha isoform. In addition, the relative transcription levels of the gene splicing variants were evaluated by RT-PCR analysis to verify a relationship with the SMO enzyme activity in various murine organs.