The Biophysical Society Annual Meeting, BPS2025 took place in the City of Angels this year, kicking off Aurora Scientific’s first conference of 2025. Unique to other disciplines, biophysics intricately explores the mechanisms of biology, including how different parts of a cell move and function, and how complex systems in our bodies work. In line with this year’s conference, the following publication review goes down the rabbit hole of biophysics discoveries, including the development of a muscle model from a rabbit fiber study, the impact of neurodevelopmental risk genes on gut physiology, and the contributions of cell tension and bulk viscoelasticity on cell stiffness.
Featured image with a photo of a white rabbit (by Francis Seura on Pexels) and components of Aurora Scientific’s 1400A Permeabilized Fiber System, including the 802D Permeabilized Fiber Apparatus, 322C High-Speed Length Controller and 403C force transducer.
Modeling thick filament activations suggests a molecular basis for force depression
Over the past 40-50 years, advances in molecular biology and biophysics have worked in tandem to further our understanding of molecular-scale events. Mathematical and computational modeling have also shaped these discoveries, contributing to a rich history of quantitative predictions, data fitting, and molecular simulations. Despite these breakthroughs, there is currently no model capable of quantitively fitting muscle measurements from the molecular to cellular scale. Recognizing this, Liu et al (2024) measured force responses in rabbit psoas muscle and developed a novel mathematical model to provide muscle contraction insights.
In preparation, single, skinned fibers were isolated from the psoas muscle of 6-month-old female New Zealand White Rabbits. Aurora Scientific’s 322C High-Speed Length Controller and 403A Force Transducer were used to measure the lengths and forces during the force-velocity and quick stretch tests. In-vitro experiments were then performed, including in-vitro motility and laser trap assays, followed by data processing and analysis.
During ramp shortening, force depression was quantified at various time points after shortening – specifically at 0.1 second, 1 second, and 2 seconds. Notably, any effect of shortening duration or velocity disappeared after 2 seconds, with the force stabilizing at only 50.8% of the pre-stretch value. On the molecular end, the in-vitro motility and laser tray assays revealed myosin step size estimations around 4.8-7.4 nm across different conditions, as well as an increased ATP binding rate with no dependence on ATP concentration. The new muscle model developed from this data was then fit to a subset of the cellular measurements, and remarkably, reasonably predicted the remaining cellular measurements, as well as the molecular measurements. This model’s ability to capture the decrease in isometric force after active shortening suggests a molecular mechanism for force depression and provides an exciting avenue for further exploration.
Kismet/CHD7/CHD8 affects gut microbiota, mechanics, and the gut-brain axis in Drosophila melanogaster
The gut-brain connection has increasingly garnered interest over the years, as mounting evidence highlights the importance of our “second brain”. Beyond “trusting our gut”, having “gut-wrenching” experiences, or feeling “butterflies in our stomach”, the complex relationship between our gut and brain are often referred to as the gut-brain axis (GBA). While research continues to link gut microbiota and brain function, how different genetic factors impact gut physiology, microbiota composition, and ultimately, neurodevelopment, remains unclear. Looking into the kismet gene of Drosophila melanogaster (an ortholog of the human neurodevelopmental disorder risk genes – CHD7 and CHD8), Niosi et al (2024) aimed to explore its impact on gut physiology and the GBA.
To conduct biomechanical measurements, full-length guts were carefully dissected from kismetLM27 null mutant and control strains, and immediately mounted between Aurora Scientific’s T-clips. The T-clips were subsequently attached to the 322C-I High-Speed Length Controller and 403B Force Transducer, enabling measurements of the gut tissue’s extension, change in length, tensile force, and linear stiffness. To assess the diversity and abundance of the gut microbiota, metagenomic 16s rRNA sequencing was performed, followed by antibiotic depletion and courtship analysis.
Upon assessment, the kismet mutant midgut had distinctly altered biomechanical properties – exhibiting significantly lower linear stiffness and significantly higher nonlinear stiffness compared to controls. Moreover, the mutant midguts also exhibited increased tensile strength, with significantly higher maximal forces and strains. Beyond biomechanics, the mutants had reduced gut microbiome diversity and abundance at every taxonomic level, with worse performance in courtship behaviours. This behavioural phenotype was rescued upon antibiotic treatment, but the resulting microbiome depletion contrastingly reduced courtship activity in control flies. As such, these findings highlight gut tissue mechanics as an important element in the gut-brain communication loop and provide crucial considerations for future studies of the GBA.
Quantifying both viscoelasticity and surface tension: Why sharp tips overestimate cell stiffness
The mechanical properties of cells, such as stiffness, elasticity, and viscosity are tightly linked to cellular function, including cell structure, division, growth, movement, adhesion, and signalling. Characterizing these properties therefore equips our understanding of healthy biological processes, as well as the changes that accompany pathological conditions such as inflammation or cancer. When measuring cell mechanics, common methods such as microindentation and micropipette aspiration are typically employed, and models either focus on cell tension (from the acto-myosin cortex) or bulk viscoelastic properties. Even so, neither approach fully captures the cell’s mechanical behaviour, and adequately representing both cell tension and viscoelasticity remains a challenge. To tackle this issue, Markova et al (2024) first developed a microindentation technique combining both surface tension and viscoelasticity, tested the approach on various cell types, and then compared it to micropipette aspiration experiments.
Cells of different sizes and stiffnesses were cultured, including T cells, PLB cells (human acute myeloid leukemia cell line), adherent and trypsinized endothelial cells (ECs) and trypsinized smooth muscle cells (SMCs). Both microindentation and micropipette aspirations techniques were employed to measure cellular mechanical properties, and Aurora Scientific’s 406A Force Transducer was used to calibrate the stiffness of the standard microindenters. After cells were indented with microindenters of different radii, a model was developed, relating cell-indenter contact stiffness to the effective Young’s modulus and surface tension.
The results revealed that the surface tension of leukocytes measured via microindentation was consistent with micropipette aspiration. In contrast, trypsinized ECs showed better agreement between the two methods when analyzed as only viscoelastic. In fact, indentation with a spherical indenter led to a small contribution of surface tension, so a better estimate of surface effects could be obtained using indentation with sharp tips. Considered together, these findings demonstrate the importance of considering surface tension in microindentation experiments for the accurate measurement of cellular mechanical properties.
Conclusions
As BPS2025 welcomed another year of exciting biophysics research, Aurora Scientific continues to provide the tools and expertise to achieve high-precision mechanical measurements in the field. These publications by Liu et al (2024), Niosi et al (2024), and Markova et al (2024) jumpstart a trail of biophysical science breakthroughs – from rabbit skinned fiber mechanics, gut-wrenching Drosophila physiology, and the precise quantification of cell stiffness.
