Cell-induced collagen alignment in a 3D in vitro culture during extracellular matrix production.

The bone extracellular matrix consists of a highly organized collagen matrix that is mineralized with carbonated hydroxyapatite. Even though the structure and composition of bone have been studied extensively, the mechanisms underlying collagen matrix organization remain elusive. In this study, we used a 3D cell culture system in which osteogenic cells deposit and orient the collagen matrix that is subsequently mineralized. Using live fluorescence imaging combined with volume electron microscopy, we visualize the organization of the cells and collagen in the cell culture. We show that the osteogenically induced cells are organizing the collagen matrix during development. Based on the observation of tunnel-like structures surrounded by aligned collagen in the center of the culture, we propose that osteoblasts organize the deposited collagen during migration through the culture. Overall, we show that cell-matrix interactions are involved in collagen alignment during early-stage osteogenic differentiation and that the matrix is organized by the osteoblasts in the absence of osteoclast activity.

Rcf1 Modulates Cytochrome c Oxidase Activity Especially Under Energy-Demanding Conditions.

The mitochondrial respiratory chain is assembled into supercomplexes. Previously, two respiratory supercomplex-associated proteins, Rcf1 and Rcf2, were identified in Saccharomyces cerevisiae, which were initially suggested to mediate supercomplex formation. Recent evidence suggests that these factors instead are involved in cytochrome c oxidase biogenesis. We demonstrate here that Rcf1 mediates proper function of cytochrome c oxidase, while binding of Rcf2 results in a decrease of cytochrome c oxidase activity. Chemical crosslink experiments demonstrate that the conserved Hig-domain as well as the fungi specific C-terminus of Rcf1 are involved in molecular interactions with the cytochrome c oxidase subunit Cox3. We propose that Rcf1 modulates cytochrome c oxidase activity by direct binding to the oxidase to trigger changes in subunit Cox1, which harbors the catalytic site. Additionally, Rcf1 interaction with cytochrome c oxidase in the supercomplexes increases under respiratory conditions. These observations indicate that Rcf1 could enable the tuning of the respiratory chain depending on metabolic needs or repair damages at the catalytic site.

Subcellular Protein Labeling by a Spatially Restricted Arylamine N-Acetyltransferase.

Mapping proteins at a specific subcellular location is essential to gaining detailed insight on local protein dynamics. We have developed an enzymatic strategy to label proteins on a subcellular level using arylamine N-acetyltransferase (NAT). The NAT enzyme activates an arylhydroxamic acid functionality into a nitrenium ion that reacts fast, covalently, and under neutral conditions with nucleophilic residues of neighboring proteins. The electron density on the aromatic ring proved important for probe activation as strong labeling was only observed with an arylhydroxamic acid bearing an electron donating substituent. We further demonstrate that, using this electron rich arylhydroxamic acid, clear labeling was achieved on a subcellular level in living cells that were transfected with a genetically targeted NAT to the nucleus or the cytosol.

Bioorthogonally Applicable Fluorogenic Cyanine-Tetrazines for No-Wash Super-Resolution Imaging.

The synthesis, fluorogenic characterization, and labeling application of four tetrazine-quenched cyanine probes with emission maxima in the red-far red range is reported. Fluorescence of the cyanine-cores is quenched via through-bond-energy-transfer (TBET) exerted by a bioorthogonal tetrazine unit. Upon bioorthogonal labeling reaction with cyclooctyne tagged proteins, the quenching effect ceases, and thus the fluorescence reinstates, resulting in an increase in fluorescence intensity. As a rare example among indocyanines, one of our new probes was found suitable in STED-based super-resolution imaging. The applicability of this fluorogenic Tet-Cy3 probe was therefore further demonstrated in the bioorthogonal labeling of cytoskeletal protein, actin, with subsequent super-resolution microscopy (STED) imaging even under no-wash conditions.