S14-Phosphorylated RPN6 Mediates Proteasome Activation by PKA and Alleviates Proteinopathy

A better understanding of the regulation of proteasome activities can facilitate the search for new therapeutic strategies. A cell culture study shows that PKA (cAMP-dependent protein kinase or protein kinase A) activates the 26S proteasome by pS14-Rpn6 (serine14-phosphorylated Rpn6), but this discovery and its physiological significance remain to be established in vivo.

Male and female mice with Ser14 of Rpn6 (regulatory particle non-ATPase 6) mutated to Ala (S14A [Rpn6/Psmd11^S14A]) or Asp (S14D) to respectively block or mimic pS14-Rpn6 were created and used along with cells derived from them. cAMP/PKA were manipulated pharmacologically. Ubiquitin-proteasome system functioning was evaluated with the GFPdgn (green fluorescence protein with carboxyl fusion of the CL1 degron) reporter mouse and proteasomal activity assays. Impact of S14A and S14D on proteotoxicity was tested in mice and cardiomyocytes overexpressing the misfolded protein R120G-CryAB (R120G [arginine120 to glycine missense mutant alpha B-crystallin]).

PKA activation increased pS14-Rpn6 and 26S proteasome activities in wild-type but not S14A embryonic fibroblasts (mouse embryonic fibroblasts), adult cardiomyocytes, and mouse hearts. Basal 26S proteasome activities were significantly greater in S14D myocardium and adult mouse cardiomyocytes than in wild-type counterparts. S14D::GFPdgn mice displayed significantly lower myocardial GFPdgn protein but not mRNA levels than GFPdgn mice. In R120G mice, a classic model of cardiac proteotoxicity, basal myocardial pS14-Rpn6 was significantly lower compared with nontransgenic littermates, which was not always associated with reduction of other phosphorylated PKA substrates. Cultured S14D neonatal cardiomyocytes displayed significantly faster proteasomal degradation of R120G than wild-type neonatal cardiomyocytes. Compared with R120G mice, S14D/S14D::R120G mice showed significantly greater myocardial proteasome activities, lower levels of total and K48-linked ubiquitin conjugates, and of aberrant CryAB (alpha B-crystallin) protein aggregates, less fetal gene reactivation, and cardiac hypertrophy, and delays in cardiac malfunction.

This study establishes in animals that pS14-Rpn6 mediates the activation of 26S proteasomes by PKA and that the reduced pS14-Rpn6 is a key pathogenic factor in cardiac proteinopathy, thereby identifying a new therapeutic target to reduce cardiac proteotoxicity.