Abstract 3211: MuRF1 and MuRF2 Are Necessary but Functionally Redundant During Developmental Cardiac Growth in vivo
Muscle ring finger (MuRF) proteins have been implicated in the transmission of mechanical forces to nuclear cell signaling pathways through their association with the sarcomere. We recently reported that MuRF1, but not MuRF2, regulates pathologic cardiac hypertrophy in vivo. This was surprising since MuRF1 and MuRF2 interact redundantly with sarcomeric proteins in yeast two hybrid studies, form both homo- and hetero-dimers, and inhibit serum response factor (SRF) activity. To determine if MuRF1 and MuRF2 are functionally redundant during cardiac development, we created mice lacking either 3 or 4 of the MuRF1 and MuRF2 alleles and compared cardiac development and activity. Surprisingly, only mice missing all four MuRF1 and MuRF2 alleles [MuRF1−/−//MuRF2−/−;MuRF1/MuRF2 double null (DN) ] were born with a massive spontaneous hypertrophic cardiomyopathy and heart failure- mice that were null for one of the genes, but heterozygous for the other (i.e. MuRF1−/−//MuRF2+/− or MuRF1+/−//MuRF2−/−) were phenotypically identical to wild type mice. Approximately 7% of the expected number of MuRF1/MuRF2 DN mice survived to adulthood, 25% dying on average post-natal day 14, and estimated ~67% dying in utero. Hearts from 12 week old MuRF1/MuRF2 DN mice had increased expression of fetal genes including smooth muscle alpha-actin, BNP, and beta-myosin heavy chain, suggesting the development of pathologic cardiac hypertrophy. Microarray analysis of genes differentially expressed between MuRF1/MuRF2 DN and wild type mice revealed a significant enrichment of genes regulated by the E2F transcription factor family. By Western immunoblot, MuRF1/MuRF2 DN hearts had significantly more E2F1 protein than either wild type mice or mice lacking 3 of the 4 MuRF1/MuRF2 alleles. These findings suggest a synergistic role for the cardiac ubiquitin ligases MuRF1 and MuRF2 proteins in regulating the activity of E2F family members. Since E2F factors play a prominent role in regulating cardiac proliferation, cell growth, and apoptosis during cardiac development, these findings suggests that MuRF1 and MuRF2 may play a redundant role in regulating E2F transcription factors essential for normal cardiac development, by regulating these fundamental biological processes.
This research has received full or partial funding support from the American Heart Association, National Center.