Abstract 706: The Apelin Receptor APJ Directly Inhibits Angiotensin II Signaling by Binding to Angiotensin II Receptor Type I and is Essential for Cardiac Development and Function
The apelin receptor (APJ) shares significant homology with the angiotensin II receptor type I (AT1R). The two receptors have similar expression profiles, including strong expression in the heart and vascular smooth muscle. Angiotensin II (ATII) and apelin have seemingly opposing roles in blood pressure regulation and vascular contractility, and AT1R and APJ likely play important roles in cardiovascular development and pathophysiology. We hypothesized that these two key pathways counter-regulate each other through molecular crosstalk. Emerging data suggest the importance of GPCR heterodimerization as a mechanism of such regulation. By performing immunoprecipitation in vascular smooth muscle cells and transfected 293 cells, we found that APJ and AT1R form stable heterodimers which are enhanced by ATII. To determine whether this heterodimerization affects receptor function, we performed co-transfections and assayed for downstream signaling activities. AT1R-transfected cells showed robust response to ATII with increase in Erk 1/2 phosphorylation and NF-kB and SRE promoter driven reporter activities, but co-transfection of APJ and AT1R resulted in marked inhibition of these responses (NF-312κB: 28.9 ± 1.2 vs 2.8 ± 0.8, SRE: 3.1 ± 0.3 vs 1.9 ± 0.03 fold induction). To investigate the role of APJ in cardiac development and function, we also developed APJ null mice. Mating between APJ heterozygotes generated expected Mendelian ratio at embryonic day 9.5 (+/+ n = 19, +/− n = 33, −/− n = 13, chi2 value = 1.1, p = 0.6) but not at birth (+/+ n = 52, +/− n = 106, −/− n = 17, chi2 value = 21.8, p < 0.0001), suggesting that over 60% of the APJ −/− mice die in utero. APJ −/− embryos showed increased heart to body mass ratio. APJ −/− mice that survive to adulthood demonstrated a significantly lower body weight (+/+ 24.75 ± 2.28, −/− 21.53 ± 1.94 grams in males, p = 0.01), as well as a markedly decreased fractional shortening via echocardiogram (+/+ 31.6 ± 3.1, −/−: 26.9 ± 3.4%, p = 0.003). Thus, we demonstrate that the absence of APJ is detrimental to cardiac development. Moreover, our findings provide a provocative mechanism by which APJ renders endogenous regulation of ATII signaling, suggesting that this crosstalk may be essential for normal cardiac development and function.