The terminal enzyme of the mitochondrial respiratory chain, cytochrome oxidase, transfers electrons to molecular oxygen, generating water. Within the inner mitochondrial membrane, cytochrome oxidase assembles into supercomplexes, together with other respiratory chain complexes, forming so-called respirasomes. Little is known about how these higher oligomeric structures are attained. Here we report on Rcf1 and Rcf2 as cytochrome oxidase subunits in S. cerevisiae. While Rcf2 is specific to yeast, Rcf1 is a conserved subunit with two human orthologs, RCF1a and RCF1b. Rcf1 is required for growth in hypoxia and complex assembly of subunits Cox13 and Rcf2, as well as for the oligomerization of a subclass of cytochrome oxidase complexes into respirasomes. Our analyses reveal that the cytochrome oxidase of mitochondria displays intrinsic heterogeneity with regard to its subunit composition and that distinct forms of respirasomes can be formed by complex variants.

Protein lysine methylation is one of the most widespread post-translational modifications in the nuclei of eukaryotic cells. Methylated lysines on histones and nonhistone proteins promote the formation of protein complexes that control gene expression and DNA replication and repair. In the cytoplasm, however, the role of lysine methylation in protein complex formation is not well established. Here we report that the cytoplasmic protein chaperone Hsp90 is methylated by the lysine methyltransferase Smyd2 in various cell types. In muscle, Hsp90 methylation contributes to the formation of a protein complex containing Smyd2, Hsp90, and the sarcomeric protein titin. Deficiency in Smyd2 results in the loss of Hsp90 methylation, impaired titin stability, and altered muscle function. Collectively, our data reveal a cytoplasmic protein network that employs lysine methylation for the maintenance and function of skeletal muscle.

Background: Titin is critical for cardiac muscle function; however, limited knowledge exists of mechanisms important for its regulation. Results: A four-and-a-half LIM domain protein-1/extracellular signal-regulated kinase-2-associated complex modulates titin-N2B levels, phosphorylation, and mechanics. Conclusion: We reveal new mechanisms underlying titin mechano-signaling. Significance: We advance our understanding of how titin-associated complexes/mutations can impact cardiac muscle function and disease.

Rationale: Cardiac tissue engineering should provide "realistic" in vitro heart muscle models and surrogate tissue for myocardial repair. For either application, engineered myocardium should display features of native myocardium, including terminal differentiation, organotypic maturation, and hypertrophic growth.Objective: To test the hypothesis that 3D-engineered heart tissue (EHT) culture supports (1) terminal differentiation as well as (2) organotypic assembly and maturation of immature cardiomyocytes, and (3) constitutes a methodological platform to investigate mechanisms underlying hypertrophic growth. Methods and Results:We generated EHTs from neonatal rat cardiomyocytes and compared morphological and molecular properties of EHT and native myocardium from fetal, neonatal, and adult rats. We made the following key observations: cardiomyocytes in EHT (1) gained a high level of binucleation in the absence of notable cytokinesis, (2) regained a rod-shape and anisotropic sarcomere organization, (3) demonstrated a fetal-to-adult gene expression pattern, and (4) responded to distinct hypertrophic stimuli with concentric or eccentric hypertrophy and reexpression of fetal genes. The process of terminal differentiation and maturation (culture days 7-12) was preceded by a tissue consolidation phase (culture days 0 -7) with substantial cardiomyocyte apoptosis and dynamic extracellular matrix restructuring. Conclusions:This study documents the propensity of immature cardiomyocytes to terminally differentiate and mature in EHT in a remarkably organotypic manner. It moreover provides the rationale for the utility of the EHT technology as a methodological bridge between 2D cell culture and animal models. (Circ Res. 2011;109:1105-1114.) Key Words: cardiac myocytes Ⅲ caspase activation Ⅲ extracellular matrix Ⅲ maturation Ⅲ hypertrophy Ⅲ sarcomere Ⅲ tissue engineering D ifferent myocardial tissue engineering formats have been developed throughout the past decade. 1 However, a low degree of cell maturation remains a key caveat in cardiac muscle engineering. A detailed understanding of "developmental" processes in tissue engineered myocardium probably is essential to guide tissue formation and maturation in vitro and to enhance the applicability of tissue engineered myocardium in substance screening, target validation, and tissue repair.Normal heart muscle growth encompasses processes of terminal differentiation and maturation by hypertrophic growth, leading to the formation of binucleated and rodshaped myocytes. 2 Physiological maturation entails a characteristic shift in gene expression, including a reduction of transcripts encoding for fetal isoforms of myofibrillar proteins while the proportion of adult isoforms increases. 3 Terminal differentiation, for example, withdrawal from the cell cycle, is another hallmark of advanced maturation already reached very early during development. 4 Cardiomyocyte monolayer cultures show neither the distinct morphological (rod-shaped) nor the molecular (adult gene expression program)...

Changes induced with transgenic cardiac HIF-1α possibly mediate beneficial effects in the short term; however, with increased mechanical load and ageing they become detrimental for cardiac function. Together with the finding of increased HIF-1α protein levels in samples from human patients with cardiomyopathy, these data indicate that chronic HIF-1α stabilization drives autonomous pathways that add to disease progression.