Identification of two splice variant forms of type-IVB cyclic AMP phosphodiesterase, DPD (rPDE-IVB1) and PDE-4 (rPDE-IVB2) in brain: selective localization in membrane and cytosolic compartments and differential expression in various brain regions

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Abstract
In order to detect the two splice variant forms of type-IVB cyclic AMP phosphodiesterase (PDE) activity, DPD (type-IVB1) and PDE-4 (type-IVB2), anti-peptide antisera were generated. One set (‘DPD/PDE-4-common’), generated against a peptide sequence found at the common C-terminus of these two PDEs, detected both PDEs. A second set was PDE-4 specific, being directed against a peptide sequence found within the unique N-terminal region of PDE-4. In brain, DPD was found exclusively in the cytosol and PDE-4 exclusively associated with membranes. Both brain DPD and PDE-4 activities, isolated by immunoprecipitation, were cyclic AMP-specific (KmcyclicAMP: approximately 5 microM for DPD; approximately 4 microM for PDE-4) and were inhibited by low rolipram concentrations (K1rolipram approximately 1 microM for both). Transient expression of DPD in COS-1 cells allowed identification of an approx. 64 kDa species which co-migrated on SDS/PAGE with the immunoreactive species identified in both brain cytosol and membrane fractions using the DPD/PDE-4-common antisera. The subunit size observed for PDE-4 (approx. 64 kDa) in brain membranes was similar to that predicted from the cDNA sequence, but that observed for DPD was approx. 4 kDa greater. Type-IV, rolipram-inhibited PDE activity was found in all brain regions except the pituitary, where it formed between 30 and 70% of the PDE activity in membrane and cytosolic fractions when assayed with 1 microM cyclic AMP, PDE-4 formed 40-50% of the membrane type-IV activity in all brain regions save the midbrain (approx. 20%). DPD distribution was highly restricted to certain regions, providing approx. 35% of the type-IV cytosolic activity in hippocampus and 13-21% in cortex, hypothalamus and striatum with no presence in brain stem, cerebellum, midbrain and pituitary. The combined type-IVB PDE activities of DPD and PDE-4 contributed approx. 10% of the total PDE activity in most brain regions except for the pituitary (zero) and the mid-brain (approx. 3%. The isolated cDNAs for DPD and PDE-4 appear to reflect transcription products which are expressed in vivo in brain. The unique N-terminal domain of PDE-4 is suggested to target this PDE to membranes in brain. Type-IVB PDEs are differentially expressed in various brain regions, indicating that there are tissue-specific controls on both the expression of the gene and the splicing of its products.

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