PKA Cß: a forgotten catalytic subunit of cAMP-dependent protein kinase opens new windows for PKA signaling and disease pathologies.

3',5'-cyclic adenosine monophosphate (cAMP) dependent protein kinase or protein kinase A (PKA) has served as a prototype for the large family of protein kinases that are crucially important for signal transduction in eukaryotic cells. The PKA catalytic subunits are encoded by the two major genes PRKACA and PRKACB, respectively. The PRKACA gene encodes two known splice variants, the ubiquitously expressed Cα1 and the sperm-specifically expressed Cα2. In contrast, the PRKACB gene encodes several splice variants expressed in a highly cell and tissue-specific manner. The Cß proteins are called Cß1, Cß2, Cß3, Cß4 and so-called abc variants of Cß3 and Cß4. Whereas Cß1 is ubiquitously expressed, Cß2 is enriched in immune cells and the Cß3, Cß4 and their abc variants are solely expressed in neuronal cells. All Cα and Cß splice variants share a kinase-conserved catalytic core and a C-terminal tail encoded by exons 2 through 10 in the PRKACA and PRKACB genes, respectively. All Cα and Cß splice variants with the exception of Cα1 and Cß1 are hyper-variable at the N-terminus. Here, we will discuss how the PRKACA and PRKACB genes have developed as paralogs that encode distinct and functionally non-redundant proteins. The fact that Cα and Cß splice variant mutations are associated with numerous diseases further opens new windows for PKA-induced disease pathologies.

The anti-inflammatory peptide Ac-SDKP is released from thymosin-ß4 by renal meprin-α and prolyl oligopeptidase.

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a natural tetrapeptide with anti-inflammatory and antifibrotic properties. Previously, we have shown that prolyl oligopeptidase (POP) is involved in the Ac-SDKP release from thymosin-ß4 (Tß4). However, POP can only hydrolyze peptides shorter than 30 amino acids, and Tß4 is 43 amino acids long. This indicates that before POP hydrolysis takes place, Tß4 is hydrolyzed by another peptidase that releases NH2-terminal intermediate peptide(s) with fewer than 30 amino acids. Our peptidase database search pointed out meprin-α metalloprotease as a potential candidate. Therefore, we hypothesized that, prior to POP hydrolysis, Tß4 is hydrolyzed by meprin-α. In vitro, we found that the incubation of Tß4 with both meprin-α and POP released Ac-SDKP, whereas no Ac-SDKP was released when Tß4 was incubated with either meprin-α or POP alone. Incubation of Tß4 with rat kidney homogenates significantly released Ac-SDKP, which was blocked by the meprin-α inhibitor actinonin. In addition, kidneys from meprin-α knockout (KO) mice showed significantly lower basal Ac-SDKP amount, compared with wild-type mice. Kidney homogenates from meprin-α KO mice failed to release Ac-SDKP from Tß4. In vivo, we observed that rats treated with the ACE inhibitor captopril increased plasma concentrations of Ac-SDKP, which was inhibited by the coadministration of actinonin (vehicle, 3.1 ± 0.2 nmol/l; captopril, 15.1 ± 0.7 nmol/l; captopril + actinonin, 6.1 ± 0.3 nmol/l; P < 0.005). Similar results were obtained with urinary Ac-SDKP after actinonin treatment. We conclude that release of Ac-SDKP from Tß4 is mediated by successive hydrolysis involving meprin-α and POP.