Structural and mechanical characterisation of the peri-prosthetic tissue surrounding loosened hip prostheses. An explorative study

Very little is known about the structure and properties of peri-prosthetic fibrous tissue that is found around loose orthopaedic implants. We describe a method for characterizing the structural organisation (histology, confocal microscopy) as well as the nano- and micro-scale mechanical behaviour (atomic force microscopy, nanoindentation) of peri-prosthetic fibrous tissue. The tissue was collected from 11 patients undergoing revision surgery due to aseptic loosening. Sirius Red and Movat histological staining procedures indicated that the tissue mainly consists of collagen fibres and ground substance. However, large inter- and intra-patient variations in the relative proportions of these tissue components were found, as well as in collagen fibre orientation and possibly also maturation. The nano-scale Young׳s moduli ranged from 0-950kPa, but showed large inter-patient variability. When the results per sample were presented in a probability density function, we could roughly discriminate one peak in the 0-100kPa range and/or one peak in the 100-500Pa range. These nano-scale moduli seem to respectively present the mechanical properties of glycosaminoglycan (GAG) and collagen molecules. The majority of the micro-scale Young׳s moduli ranged between 0.5 and 2.0kPa for all samples. This explorative study provides new insights in (the variations of) structural organisation and mechanical properties of peri-prosthetic tissue.

Unraveling the role of the rssC gene of Serratia marcescens by atomic force microscopy

The product and direct role of the rssC gene of Serratia marcescens is unknown. For unraveling the role of the rssC gene, atomic force microscopy has been used to identify the surfaces of intact S. marcescens wild-type CH-1 cells and rssC mutant CH-1ΔC cells. The detailed surface topographies were directly visualized, and quantitative measurements of the physical properties of the membrane structures were provided. CH-1 and CH-1ΔC cells were observed before and after treatment with lysozyme, and their topography-related parameters, e.g., a valley-to-peak distance, mean height, surface roughness, and surface root-mean-square values, were defined and compared. The data obtained suggest that the cellular surface topography of mutant CH-1ΔC becomes rougher and more precipitous than that of wild-type CH-1 cells. Moreover, it was found that, compared with native wild-type CH-1, the cellular surface topography of lysozyme-treated CH-1 was not changed profoundly. The product of the rssC gene is thus predicted to be mainly responsible for fatty-acid biosynthesis of the S. marcescens outer membrane. This study represents the first direct observation of the structural changes in membranes of bacterial mutant cells and offers a new prospect for predicting gene expression in bacterial cells.

Combining mechanical and optical approaches to dissect cellular mechanobiology

Mechanical force modulates a wide array of cell physiological processes. Cells sense and respond to mechanical stimuli using a hierarchy of structural complexes spanning multiple length scales, including force-sensitive molecules and cytoskeletal networks. Understanding mechanotransduction, i.e., the process by which cells convert mechanical inputs into biochemical signals, has required the development of novel biophysical tools that allow for probing of cellular and subcellular components at requisite time, length, and force scales and technologies that track the spatio-temporal dynamics of relevant biomolecules. In this review, we begin by discussing the underlying principles and recent applications of atomic force microscopy, magnetic twisting cytometry, and traction force microscopy, three tools that have been widely used for measuring the mechanical properties of cells and for probing the molecular basis of cellular mechanotransduction. We then discuss how such tools can be combined with advanced fluorescence methods for imaging biochemical processes in living cells in the context of three specific problem spaces. We first focus on fluorescence resonance energy transfer, which has enabled imaging of intra- and inter-molecular interactions and enzymatic activity in real time based on conformational changes in sensor molecules. Next, we examine the use of fluorescence methods to probe force-dependent dynamics of focal adhesion proteins. Finally, we discuss the use of calcium ratiometric signaling to track fast mechanotransductive signaling dynamics. Together, these studies demonstrate how single-cell biomechanical tools can be effectively combined with molecular imaging technologies for elucidating mechanotransduction processes and identifying mechanosensitive proteins.