Visualization of cardiac thick filament dynamics in ex vivo heart preparations

RATIONALE

Cardiac muscle cells are terminally differentiated after birth and must beat continually throughout one's lifetime. This mechanical process is driven by the sliding of actin-based thin filaments along myosin-based thick filaments, organized within sarcomeres. Despite costly energetic demand, the half-life of the proteins that comprise the cardiac thick filaments is ∼10 days, with individual molecules being replaced stochastically, by unknown mechanisms.

OBJECTIVES

To allow for the stochastic replacement of molecules, we hypothesized that the structure of thick filaments must be highly dynamic in vivo.

METHODS AND RESULTS

To test this hypothesis in adult mouse hearts, we replaced a fraction of the endogenous myosin regulatory light chain (RLC), a component of thick filaments, with GFP-labeled RLC by adeno-associated viral (AAV) transduction. The RLC-GFP was properly localized to the heads of the myosin molecules within thick filaments in ex vivo heart preparations and had no effect on heart size or actin filament siding in vitro. However, the localization of the RLC-GFP molecules was highly mobile, changing its position within the sarcomere on the minute timescale, when quantified by fluorescence recovery after photobleaching (FRAP) using multiphoton microscopy. Interestingly, RLC-GFP mobility was restricted to within the boundaries of single sarcomeres. When cardiomyocytes were lysed, the RLC-GFP remained strongly bound to myosin heavy chain, and the intact myosin molecules adopted a folded, compact configuration, when disassociated from the filaments at physiological ionic conditions.

CONCLUSIONS

These data demonstrate that the structure of the thick filament is highly dynamic in the intact heart, with a rate of molecular exchange into and out of thick filaments that is ∼1500 times faster than that required for the replacement of molecules through protein synthesis or degradation.

Aiming for the top: non-cell autonomous control of shoot stem cells in Arabidopsis

In multicellular organisms, not all cells are created equal. Instead, organismal complexity is achieved by specialisation and division of labour between distinct cell types. Therefore, the organism depends on the presence, correct proportion and function of all cell types. It follows that early development is geared towards setting up the basic body plan and to specify cell lineages. Since plants employ a post-embryonic mode of development, the continuous growth and addition of new organs require a source of new cells, as well as a strict regulation of cellular composition throughout the entire life-cycle. To meet these demands, evolution has brought about complex regulatory systems to maintain and control continuously active stem cell systems. Here, we review recent work on the mechanisms of non cell-autonomous control of shoot stem cells in the model plant Arabidopsis thaliana with a strong focus on the cell-to-cell mobility and function of the WUSCHEL homeodomain transcription factor.

Time-shifted mean-segmented Q data of a luminal protein measured at the nuclear envelope by fluorescence fluctuation microscopy

Fluorescence fluctuation microscopy is a widely used method to determine the mobility and oligomeric state of proteins in the live cell environment. Existing analysis methods rely on statistical evaluation of data segments with the implicit assumption that no significant signal fluctuations occur on the time scale of a data segment. Recent work on extending fluorescence fluctuation methods to the nuclear envelope of living cells identified a slow fluctuation process that is associated with the undulations of the nuclear membranes, which lead to intensity fluctuations due to local volume changes at the nuclear envelope. This environment violates the above-mentioned assumption and is associated with biased evaluation of fluorescence fluctuation data by traditional analysis methods, such as the autocorrelation function. This challenge was overcome by the introduction of the time-shifted mean-segmented Q function, which relies on a sliding scale of data segment lengths. Here, we share experimental fluorescence fluctuation data taken at the nuclear envelope and demonstrate the calculation of the time-shifted mean-segmented Q function from the raw data. The data and analysis should be valuable for researchers interested in fluorescence fluctuation techniques and provides an opportunity to examine the influence of slow fluctuations on existing data analysis methods. The data is related to the research article titled "Protein oligomerization and mobility within the nuclear envelope evaluated by the time-shifted mean-segmented Q factor" [1].

Dynamic origins of substrate promiscuity in bacterial galactokinases

Galactokinase catalyses the ATP-dependent phosphorylation of galactose and structurally related sugars. The enzyme has attracted interest as a potential biocatalyst for the production of sugar 1-phosphates and several attempts have been made to broaden its specificity. In general, bacterial galactokinases have wider substrate ranges than mammalian ones. The enzymes from Escherichia coli and Lactococcus lactis have received particular attention and a number of variants with increased promiscuity have been identified. Here, we present a molecular dynamics study designed to investigate the molecular causes of the wider substrate ranges of these enzymes and their variants with particular reference to protein mobility. Some regions close to the active site of the enzyme have different structures in the bacterial enzymes compared to the human one. Alterations known to increase the substrate range (e.g. Y371H in the E. coli enzyme), tend to alter the conformation of a key α-helical region (residues 216-232 in the E. coli enzyme). The equivalent helix in the human enzyme has previously been predicted to be altered in variants which affect catalytic activity or protein stability. This helix appears to be a key region in galactokinases from a range of species and may represent an interesting target for future attempts to broaden the specificity of galactokinases.

The strength of the protein-material interaction determines cell fate

Extracellular matrix (ECM) proteins are key mediators of cell/material interactions. The surface density and conformation of these proteins adsorbed on the material surface influence cell adhesion and the cellular response. We have previously shown that subtle variations in surface chemistry lead to drastic changes in the conformation of adsorbed fibronectin (FN). On poly(ethyl acrylate) (PEA), FN unfolds and displays domains for cell adhesion and FN-FN interaction, whereas on poly(methyl acrylate) (PMA) - with only one methyl group less - FN remains globular as it is in solution. The effect of the strength of the protein/material interaction in cell response, and its relation to protein density and conformation, has received limited attention so far. In this work, we used FN-functionalized AFM cantilevers to evaluate, via force spectroscopy, the strength of interaction between fibronectin and the underlying polymer which controls FN conformation (PEA and PMA). We found that the strength of FN/PEA interaction is significantly higher than FN/PMA, which limits the mobility of FN layer on PEA, reduces the ability of cells to mechanically reorganize FN and then leads to enhanced proteolysis and degradation of the surrounding matrix with compromised cell viability. By contrast, both PEA and PMA support cell adhesion when FN density is increased and also in the presence of serum or other serum proteins, including vitronectin (VN) and bovine serum albumin (BSA), which provide a higher degree of mobility to the matrix.

STATEMENT OF SIGNIFICANCE

The identification of parameters influencing cell response is of paramount importance for the design of biomaterials that will act as synthetic scaffolds for cells to anchor, grow and, eventually, become specialised tissues. Cells interact with materials through an intermediate layer of proteins adsorbed on the material surface. It is known that the density and conformation of these proteins determine cell behaviour. Here we show that the strength of protein/material interactions, which has received very limited attention so far, is key to understand the cellular response to biomaterials. Very strong protein/material interactions reduce the ability of cells to mechanically reorganize proteins at the material interface which results in enhanced matrix degradation, leading ultimately to compromised cell viability.