Identification and characterization of novel mouse PDE4D isoforms: molecular cloning, subcellular distribution and detection of isoform-specific intracellular localization signals.

We report here the cloning and characterization of short and supershort mouse PDE4D isoforms. PDE4D is one of the phosphodiesterase enzyme families with multiple promoters and splice variants. PDE4 isoforms present in humans, rats and mice share considerable homology in their catalytic and regulatory domains. In this study, we have identified the novel PDE4D2 variant3 (PDE4D2v3) and PDE4D10 isoforms and the mouse orthologs of PDE4D1, PDE4D2 variant1 (PDE4D2v1), PDE4D2 variant2 (PDE4D2v2) and PDE4D6 isoforms. These isoforms have many different lengths of 5'UTR, signifying the use of different transcription start sites. Our data indicate that many novel PDE4D isoforms exist as a result of alternative mRNA splicing, each isoform having unique N-terminal regions and multiple transcription start sites. Subcellular distribution study showed that the PDE4D1 short isoforms are localized to the nucleus while the supershort isoforms (PDE4D2v1, PDE4D2v2, PDE4D2v3, PDE4D6 and PDE4D10) are restricted to the cytoplasm. Deletion study confirmed that the N-terminus of PDE4D1 is necessary for nuclear targeting. In addition, we showed that the unique N-terminus contains nuclear localization signal sequence. Identifying novel tissue-specific PDE4D isoforms with unique N-terminal regions may aid in the development of selective phosphodiesterase inhibitors.