A cost-effective method to immobilize hydrated soft-tissue samples for atomic force microscopy

Microindentation of fresh soft biological tissue: A rapid tissue sectioning and mounting protocol

Microindentation of fresh biological tissues is necessary for the creation of 3D biomimetic models that accurately represent the native extracellular matrix microenvironment. However, tissue must first be precisely sectioned into slices. Challenges exist in the preparation of fresh tissue slices, as they can tear easily and must be processed rapidly in order to mitigate tissue degradation. In this study, we propose an optimised mounting condition for microindentation and demonstrate that embedding tissue in a mixture of 2.5% agarose and 1.5% gelatin is the most favourable method of tissue slice mounting for microindentation. This protocol allows for rapid processing of fresh biological tissue and is applicable to a variety of tissue types.

Microenvironmental stiffness induces metabolic reprogramming in glioblastoma

The mechanical properties of solid tumors influence tumor cell phenotype and the ability to invade surrounding tissues. Using bioengineered scaffolds to provide a matrix microenvironment for patient-derived glioblastoma (GBM) spheroids, this study demonstrates that a soft, brain-like matrix induces GBM cells to shift to a glycolysis-weighted metabolic state, which supports invasive behavior. We first show that orthotopic murine GBM tumors are stiffer than peritumoral brain tissues, but tumor stiffness is heterogeneous where tumor edges are softer than the tumor core. We then developed 3D scaffolds with μ-compressive moduli resembling either stiffer tumor core or softer peritumoral brain tissue. We demonstrate that the softer matrix microenvironment induces a shift in GBM cell metabolism toward glycolysis, which manifests in lower proliferation rate and increased migration activities. Finally, we show that these mechanical cues are transduced from the matrix via CD44 and integrin receptors to induce metabolic and phenotypic changes in cancer cells.

Micromechanical characterizations and viscoelastic modeling reveal elastic and viscoelastic heterogeneities in ovarian tissue and the significant viscoelastic contribution to the apparent elastic modulus determined by AFM indentation

Ovarian follicles develop in a highly regulated mechanical microenvironment and disruptions to the microenvironment may cause infertility. However, the viscoelastic properties of the ovarian tissue are not well studied. Here, we characterize both the elastic and viscoelastic properties of ovarian tissue from both reproductively older and younger domestic cats using atomic force microscopy (AFM) indentation and viscoelastic models of stress relaxation. Importantly, our analyses reveal the apparent elastic modulus obtained from the conventional AFM indentation measurement is significantly higher than the intrinsic elastic modulus and insignificantly different from the equivalent elastic modulus that is the summation of the intrinsic elastic modulus and the viscoelastic contribution to modulus at time 0. Interestingly, the ovarian cortex of both reproductive age groups has a higher apparent/intrinsic modulus than that of the medulla. Furthermore, two different kinetics of stress relaxation are identified with rate constants of ∼1 s and ∼20-40 s, respectively. Moreover, the rate constant of the slow kinetics is significantly different between the cortex and medulla in the reproductively older ovaries. Finally, these mechanical heterogeneities appear to follow the heterogeneous distribution of hyaluronic acid (HA) in the ovary. These findings may be invaluable to the development of biomimetic follicle culture for treating infertility. STATEMENT OF SIGNIFICANCE: This study investigates not only elastic but also the viscoelastic heterogeneity in both reproductively younger and older ovarian tissues for the first time. Further, by combining AFM indentation measurement and viscoelastic modeling, we show the apparent elastic modulus conventionally reported in the literature for AFM indentation measurement is the summation of the intrinsic elastic modulus and a significant viscoelastic contribution to the modulus at time 0. This is an important consideration for others who use this method to quantify biomaterial properties. In addition, the possible connection between the mechanical and compositional heterogeneities is explored. These findings may be invaluable for designing biomaterials to recapitulate the mechanical environment of the ovary and possibly many other organs for biomimetic tissue engineering.

Self-assembly of a barnacle cement protein into intertwined amyloid fibres and determination of their adhesive and viscoelastic properties

The stalked barnacle Pollicipes pollicipes uses a multi-protein cement to adhere to highly varied substrates in marine environments. We investigated the morphology and adhesiveness of a component 19 kDa protein in barnacle cement gland- and seawater-like conditions, using transmission electron microscopy and state-of-the art scanning probe techniques. The protein formed amyloid fibres after 5 days in gland-like but not seawater conditions. After 7-11 days, the fibres self-assembled under gland-like conditions into large intertwined fibrils of up to 10 µm in length and 200 nm in height, with a distinctive twisting of fibrils evident after 11 days. Atomic force microscopy (AFM)-nanodynamic mechanical analysis of the protein in wet conditions determined E' (elasticity), E'' (viscosity) and tan δ values of 2.8 MPa, 1.2 MPa and 0.37, respectively, indicating that the protein is a soft and viscoelastic material, while the adhesiveness of the unassembled protein and assembled fibres, measured using peak force quantitative nanomechanical mapping, was comparable to that of the commercial adhesive Cell-Tak™. The study provides a comprehensive insight into the nanomechanical and viscoelastic properties of the barnacle cement protein and its self-assembled fibres under native-like conditions and may have application in the design of amyloid fibril-based biomaterials or bioadhesives.

Novel Decellularization Method for Tissue Slices

Decellularization procedures have been developed and optimized for the entire organ or tissue blocks, by either perfusion of decellularizing agents through the tissue’s vasculature or submerging large sections in decellularizing solutions. However, some research aims require the analysis of native as well as decellularized tissue slices side by side, but an optimal protocol has not yet been established to address this need. Thus, the main goal of this work was to develop a fast and efficient decellularization method for tissue slices—with an emphasis on lung—while attached to a glass slide. To this end, different decellularizing agents were compared for their effectiveness in cellular removal while preserving the extracellular matrix. The intensity of DNA staining was taken as an indicator of remaining cells and compared to untreated sections. The presence of collagen, elastin and laminin were quantified using immunostaining and signal quantification. Scaffolds resulting from the optimized protocol were mechanically characterized using atomic force microscopy. Lung scaffolds were recellularized with mesenchymal stromal cells to assess their biocompatibility. Some decellularization agents (CHAPS, triton, and ammonia hydroxide) did not achieve sufficient cell removal. Sodium dodecyl sulfate (SDS) was effective in cell removal (1% remaining DNA signal), but its sharp reduction of elastin signal (only 6% remained) plus lower attachment ratio (32%) singled out sodium deoxycholate (SD) as the optimal treatment for this application (6.5% remaining DNA signal), due to its higher elastin retention (34%) and higher attachment ratio (60%). Laminin and collagen were fully preserved in all treatments. The SD decellularization protocol was also successful for porcine and murine (mice and rat) lungs as well as for other tissues such as the heart, kidney, and bladder. No significant mechanical differences were found before and after sample decellularization. The resulting acellular lung scaffolds were shown to be biocompatible (98% cell survival after 72 h of culture). This novel method to decellularize tissue slices opens up new methodological possibilities to better understand the role of the extracellular matrix in the context of several diseases as well as tissue engineering research and can be easily adapted for scarce samples like clinical biopsies.