Dual role of tropomyosin on chemically modified actin filaments from skeletal muscle

Distinct niches within the extracellular matrix dictate fibroblast function in (cell free) 3D lung tissue cultures

Cues from the extracellular matrix (ECM) and their functional interplay with cells play pivotal roles for development, tissue repair, and disease. However, the precise nature of this interplay remains elusive. We used an innovative 3D cell culture ECM model by decellularizing 300-µm-thick ex vivo lung tissue scaffolds (d3D-LTCs) derived from diseased and healthy mouse lungs, which widely mimics the native (patho)physiological in vivo ECM microenvironment. We successfully repopulated all d3D-LTCs with primary human and murine fibroblasts, and moreover, we demonstrated that the cells also populated the innermost core regions of the d3D-LTCs in a real 3D fashion. The engrafted fibroblasts revealed a striking functional plasticity, depending on their localization in distinct ECM niches of the d3D-LTCs, affecting the cells’ tissue engraftment, cellular migration rates, cell morphologies, and protein expression and phosphorylation levels. Surprisingly, we also observed fibroblasts that were homing to the lung scaffold’s interstitium as well as fibroblasts that were invading fibrotic areas. To date, the functional nature and even the existence of 3D cell matrix adhesions in vivo as well as in 3D culture models is still unclear and controversial. Here, we show that attachment of fibroblasts to the d3D-LTCs evidently occurred via focal adhesions, thus advocating for a relevant functional role in vivo. Furthermore, we found that protein levels of talin, paxillin, and zyxin and phosphorylation levels of paxillin Y118, as well as the migration-relevant small GTPases RhoA, Rac, and CDC42, were significantly reduced compared with their attachment to 2D plastic dishes. In summary, our results strikingly indicate that inherent physical or compositional characteristics of the ECM act as instructive cues altering the functional behavior of engrafted cells. Thus, d3D-LTCs might aid to obtain more realistic data in vitro, with a high relevance for drug discovery and mechanistic studies alike.

Protein Expression of STRO-1 Cells in Response to Different Topographic Features

Human skeletal stem cells (STRO-1 positive) display distinct responses to different topographical features. On a flat surface, skeletal cells spread, and in vitro, they typically display a polarized, fibroblast-like morphology. However, on microgrooved surfaces, these cells prefer to stretch along the grooves forming a similar morphology to in vivo, bipolarized fibroblasts. In contrast, on nanopits, these cells display a polygonal and osteoblastic phenotype. We have examined mechanotransduction events of STRO-1 positive in response to fibroblastic, microgrooved and osteogenic, controlled disorder nanopit, topographies using proteomics after 3 days in culture. Protein expression profiles were analyzed by difference gel electrophoresis to identify proteins that showed modulation of expression in response to different topographic features to assess early decision events in these cells on these discrete topographies. After only 72 hours in culture, STRO-1 positive displayed differential regulations of families of proteins involved in cell migration and proliferation. The current study indicated that osteogenic decision specific events had already occurred. Runx2 was localized in nuclei of the skeletal stem cells on the osteogenic nanopits; however, few signaling pathway changes were observed. This study demonstrated that micro- and nanotopographies activated skeletal stem cells at different times and with distinct mechanotransduction profiles.

Troponin Is a Potential Regulator for Actomyosin Interactions

Troponin extracted from rabbit skeletal muscle directly binds to an actin filament in a molar ratio of 1:1 even in the absence of tropomyosin. An actin filament decorated with troponin did not exhibit significant difference from pure actin filaments in the maximum rate of actomyosin ATP hydrolysis and the sliding velocity of the filament examined by means of an in vitro motility assay. However, the relative number of troponin-bound actin filaments moving in the absence of calcium ions decreased to half that in their presence. The amount of HMM bound to the filaments was less than 4% of actin monomers in the presence of TNs. In addition, actin filaments could not move when Tn molecules were bound in the molar ratio of about 1:1 although they sufficiently bind to myosin heads. These results indicate that troponin can transform an actin monomer within a filament into an Off-state without sterically blocking of the myosin-binding sites with tropomyosin molecules.

Significance of kinetic degrees of freedom in operation of the actomyosin motor

The actomyosin motor as a principal functional component of cell motility is highly coordinated in regulating the participating molecular components. At the same time, it has to be flexible and plastic enough to accommodate itself to a wide variety of operational conditions. We prepared two different types of actomyosin systems. One is a natural intact actomyosin system with no artificial constraint on the kinetic degrees of freedom of the actin filaments, and the other is a regulated one with actin filaments supplemented by intra- and intermolecular crosslinking to suppress the kinetic degrees of freedom to a certain extent. Crosslinked actomyosin systems were found to remain almost insensitive to calcium regulation even when intact troponin-tropomyosin regulatory component was incorporated. Both the ATPase and the motile activities of the actin filaments sliding on myosin molecules were markedly lowered by the crosslinking. In contrast, once the crosslinking was cleaved, both properties returned to the normal as with intact actomyosin systems.

Characterization of three regulatory states of the striated muscle thin filament

The troponin-tropomyosin-linked regulation of striated muscle contraction occurs through allosteric control by both Ca(2+) and myosin. The thin filament fluctuates between two extreme states: the inactive "off" state and the active "on" state. Intermediate states have been proposed from structural studies and transient kinetic measurements. However, in contrast to the well-characterised, on and off states, the mechanochemical properties of the intermediate states are much less well understood because of the instability of those states. In the present study, we have characterized a myosin-induced intermediate that is stabilized by cross-linking myosin motor domains (S1) to actin filaments (with a maximum of one S1 molecule for 50 actin monomers). A single S1 molecule is known to interact with two adjacent actin monomers. A detailed analysis revealed that thin filaments containing S1 molecules cross-linked to just one actin monomer (actin(1)-S1 complexes) are regulated with a 79% inhibition of the ATPase in the absence of Ca(2+). In contrast, filaments containing S1 molecules cross-linked at two positions, to two adjacent actin monomers (actin(2)-S1 complexes) totally lose their regulation in a highly cooperative manner. This loss of regulation was due both to an enhancement of the ATPase activity without calcium and an inhibition of the ATPase with calcium. Filaments containing actin(2)-S1 complexes, with significant ATPase activity in the absence of calcium (about 50%), did not move on a myosin-coated surface unless calcium was present. This partial uncoupling between the ATPase activity and in vitro motility in the absence of calcium demonstrates that the mechanical steps require actin-myosin contacts, which take place only in the on state and not in the off or intermediate states. These data provide new insights concerning the difference in cooperativity of Ca(2+) regulation that exists between the biochemical and mechanical cycles of the actin-myosin motor.

Tropomyosin directly modulates actomyosin mechanical performance at the level of a single actin filament

Muscle contraction is the result of myosin cross-bridges (XBs) cyclically interacting with the actin-containing thin filament. This interaction is modulated by the thin filament regulatory proteins, troponin and tropomyosin (Tm). With the use of an in vitro motility assay, the role of Tm in myosin's ability to generate force and motion was assessed. At saturating myosin surface densities, Tm had no effect on thin filament velocity. However, below 50% myosin saturation, a significant reduction in actin-Tm filament velocity was observed, with complete inhibition of movement occurring at 12. 5% of saturating surface densities. Under similar conditions, actin filaments alone demonstrated no reduction in velocity. The effect of Tm on force generation was assessed at the level of a single thin filament. In the absence of Tm, isometric force was a linear function of the density of myosin on the motility surface. At 50% myosin surface saturation, the presence of Tm resulted in a 2-fold enhancement of force relative to actin alone. However, no further potentiation of force was observed with Tm at saturating myosin surface densities. These results indicate that, in the presence of Tm, the strong binding of myosin cooperatively activates the thin filament. The inhibition of velocity at low myosin densities and the potentiation of force at higher myosin densities suggest that Tm can directly modulate the kinetics of a single myosin XB and the recruitment of a population of XBs, respectively. At saturating myosin conditions, Tm does not appear to affect the recruitment or the kinetics of myosin XBs.

Staggered movement of an actin filament sliding on myosin molecules in the presence of ATP

An actin filament sliding on myosin molecules in the presence of an extremely low concentration of ATP exhibited a staggered movement. Longitudinally sliding movement of the filament was frequently interrupted by its non-sliding, fluctuating movements both in the longitudinal and transversal directions. Intermittent sliding movements of an actin filament indicate establishment of a coordination of ATP-mediated active sites distributed along the filament.