The role of Eukaryotic elongation factor 1 α in cardiac homeostasis

Background: The canonical function of Eukaryotic elongation factor 1 α (Eef1a) is the translocation of tRNA from the cytosol to the ribosome during translational elongation. In addition, Eef1a is linked to F-actin formation, proteasome activity, aggresome assembly and microtubules formation. In mammalian cells, there are two paralogs of Eef1a: Eef1a1 and Eef1a2. Whereas Eef1a1 is ubiquitously expressed in every cell type, the expression of Eef1a2 is restricted to adult cardiomyocytes, skeletal myocytes and neurons.  Recently, it was shown that patients with mutations in Eef1a2 develop cardiomyopathies; however, the mechanism involved is completely unknown.

Methods and Results: Here we generated cardiomyocyte specific, tamoxifen induced adult onset knock-out mice for Eef1a1 (eEf1a1 cKO), Eef1a2 (eEf1a2 cKO), and for Eef1a1 and Eef1a2 in combination (eEf1a1/a2 cKO).  We analyzed cardiac dimensions and function by echocardiography, morphometry and conducted histological analyses of heart tissue. In addition, we analyze how Eef1a2 acts in cardiomyocytes on the molecular level. Our result show that Eef1a2 cKO exert decreased cardiac function after 2 months post gene-deletion, but not at 1-month post deletion. This decrease was accompanied by an increase in ventricular weight, cardiomyocyte cross-sectional area and interstitial fibrosis. 40% of Eef1a2 cKO mice died within 2 months post deletion. By contrast, Eef1a1 cKO had normal cardiac function after 2 months of gene-deletion and no premature mortality. The Eef1a1/a2 cKO mice exerted an early mortality (40 days post deletion), however, the cardiac function remained unaltered, suggesting sudden death at that time point. Mechanistically, we observed an increased in the translational efficiency (TE) of ribosomal subunits and elongation factors by Ribo Seq at 1-month post tamoxifen, suggesting that the alteration in the translation of these genes is an early event. Interesting, the global cardiac protein synthesis rate, assessed by Puromycin incorporation, was similar between Eef1a2 cKO and WT mice, and it was only reduced by 30% in Eef1a1/a2 cKO mice 1-month post deletion. Altogether our data suggest that the increase in the TE of ribosomal subunits might function as compensatory effect. Analysis of Eef1a2-cKO after 2 months post tamoxifen showed an increase in cardiomyocytes with accumulation of misfolded proteins and p62.  Finally, we assessed the possibility that Eef1a2 might function as a chaperone using luciferase activity assays. Overexpression of luciferase in adult cardiomyocytes from Eef1a2-cKO or Eef1a1/a2-cKO show a strong reduction in the luciferase activity without alteration in the mRNA expression, suggesting that Eef1a2 might function as a chaperone in cardiomyocytes.

Conclusions:  Sufficient eEF1A levels in cardiomyocytes are required to maintain cardiac homeostasis and function. Our results demonstrate for the first time that eEF1A levels regulate the homeostasis of ribosome subunits expression in cardiomyocytes, and that eEF1A1 functions are partially redundant with eEF1A2. We propose that cardiac Eef1a2 is critical for the proper folding of proteins, and alterations in the levels of Eef1a2 thereby have a great impact on proteostasis, independent of global protein synthesis.