| Ornithine decarboxylase: Expression and regulation in rat brain and in transgenic mice | ||
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In this study we wanted to elucidate the role of the first and crucial enzyme of polyamine biosynthesis, i.e. ornithine decarboxylase, and its protein inhibitor, antizyme, in the polyamine metabolism of CNS. In particular we were interested in whether the guanosine 5’-phosphate activatable ODC activity previously detected in mammalians only in some tumours, could be found in CNS as we hypothesized. We were able to show that ODC in mammalian brain lysates is activated by GTP, and that this activation is more significant after antizyme is separated from ODC in the gel filtration chromatography. GTPactivatable ODC was more resistant to heat denaturation and displayed higher Vmax than kidney ODC. We continued our studies by localizing ODC and antizyme expression in the adult rat brain using both in situ hybridization and immunocytochemistry. We detected wide and mostly overlapping expression patterns. Interestingly, we observed that in some areas of brains antizyme had mainly nuclear localization. In addition, we mutated aspartate-233 to valine in mouse ODC. The mutation increased Km values for the cofactor PLP and the substrate L-ornithine as well as Ki value for the inhibitor DFMO.
Another major aim of this study was to generate transgenic mouse line expressing ODC cDNA under the control of a MMTV-LTR promotor, and to use it to study the pathological and physiological effects of deregulated ODC expression during the life of transgenic animals. We detected the most significant changes in ODC expression in reproductive organs of male mice. The number of infertile mice was high supporting earlier reports about the importance of balanced polyamine metabolism for normal spermatogenesis. The involvement of polyamines in the fertility of females remained unproven. Transgenic mice were prone to various pathological conditions, which may be due to deregulated polyamine methabolism.
The most interesting results of this study were the detection of GTP-activatable ODC in normal mammalian tissue and the detection of the mainly nuclear localization for antizyme in some areas of brain. These both observations were reported for the first time in our study. The latter has already confirmed by others and may imply together with some other recent observations that the antizyme is much more versatile and multifunctional protein than recognised still few years ago.