The line tension of the pore in a phospholipid bilayer is important for pore-mediated molecular transport techniques. To understand the cholesterol effects on the line tension of the pore edge at the molecular level, we perform molecular dynamics simulations of phospholipid bilayers with a pore containing cholesterol in different concentrations (0, 20, and 40 mol%). The bilayer with a pore is prepared by using an equibiaxial stretching simulation. The stretched bilayer with a pore is subsequently compressed and the pore spontaneously closes when the applied areal strain of the bilayer is below a certain value. Using the pore closure areal strain and a free energy model of a stretched bilayer with a pore, the upper and lower limits of the line tensions for the bilayers containing cholesterol at 0, 20, and 40 mol% are estimated to be 17.0-48.2, 54.5-100, and 170-261 pN, respectively. The increasing tendency of the line tension qualitatively agrees with that observed experimentally. The pores in the cholesterol-containing bilayers are lined with several cholesterol molecules, which might increase the bending rigidity of the pore edge, and result in the higher line tension of the cholesterol-containing bilayer. The considerable dependency of the line tension on the bilayer compositions might be useful to explain the large variations of the transduction efficiency observed with sonoporation treatment.

Keywords

Cell membrane, Sonoporation, Electroporation, DPPC bilayer, Membrane rupture, Pore closure

Paper information

Taiki SHIGEMATSU, Kenichiro KOSHIYAMA, Shigeo WADA, “Line tension of the pore edge in phospholipid/cholesterol bilayer from stretch molecular dynamics simulation”, Journal of Biomechanical Science and Engineering, Vol.11, No.1 (2016), p.15-00422. doi:10.1299/jbse.15-00422. Final Version Released on June 17, 2016, Advance Publication Released on October 16, 2015.

In order to determine how cells change their morphology during adhesion process to a substrate, we focused on the actin cytoskeleton and investigated its morphological change along with that of the whole cell during adhesion process. An osteoblastic cell line MC3T3-E1 was used as the test model. We plated cells whose cell cycle had been synchronized by serum starvation on fibronectin-coated glass plate and cultured them for 10 min to 24 h. We then stained their F-actin and nucleus and observed them with a fluorescent microscope for cell area and shape index and 2D parameters for actin morphology, and with a laser scanning microscope for 3D morphology of actin and nucleus. In the beginning of adhesion, the trypsinized cells were round and their nuclei were surrounded uniformly by thick layer of actin. The actin layer in the upper side became actin aggregate (AA) and lower side dense peripheral band (DPB) in 30 min. The upper AA then became smaller and finally to actin filaments (AFs) spanning the cell top. The DPB expanded and finally became AFs on cell bottom by 1 h. The nucleus becomes flattened possibly due to compression by the cell membrane caused by the expansion of the DPB in the early stage of adhesion. In the later stage of adhesion, the number of AFs continuously increased and nucleus became flattened more and more until 12 h. This may be caused by the increase in the top AFs that may compress the nucleus. Cells become more elongated in response to further alignment of AFs until 12 h. These results indicate that change in AFs during adhesion process is complicated not only temporally but also spatially.

Keywords

Cytoskeleton, Actin aggregate, Actin filament, Morphology, MC3T3-E1, Adhesion

Paper information

Junfeng WANG, Shukei SUGITA, Kazuaki NAGAYAMA, Takeo MATSUMOTO, “Dynamics of actin filaments of MC3T3-E1 cells during adhesion process to substrate”, Journal of Biomechanical Science and Engineering, Vol.11, No.2 (2016), p.15-00637. doi:10.1299/jbse.15-00637. Final Version Released on June 24, 2016, Advance Publication Released on January 04, 2016.

We previously described an analytical model regarding how human falsetto voice is produced upon interaction between the respiratory airflow and vocal fold motion. This theory highlights the role of an unsteady flow effect―or specifically, convective acceleration of wall motion-induced flow―in inducing a Hopf bifurcation or aerodynamic flutter of vocal folds, reminiscent of falsetto voice production. Importantly, the mucosal wave motion and glottal closure of the vocal folds―typically observed in human modal voice production but absent in falsetto vocalization of high voice pitch―are dispensable in this analytical model. Thus, given its rigorous applicability to high-pitched vocalization, our model may function as a universal physical mechanism underlying the vocalization of not only humans but also other diverse vertebrate animals that share a basic anatomical design. Here we show that the relationship between the vocal frequency and animal body size and mass, derived from the present model, captures the actual features reported elsewhere, thus suggesting that the allometric scaling of animal vocalization is explained theoretically. Moreover, the critical biomechanical conditions that induce the vocalization are rewritten to highlight an intriguing consequence from our model that the voice pitch can be controlled simply and extensively with the mechanical tension in vocal membranes. Furthermore, several dimensionless numbers that characterize the aerodynamic flutter are introduced to shed light on the physical essence of the possible universal mechanism underlying vertebrate animal vocalization: whether the animal prefers to falsetto or modal vocalization is determined by its communication frequency.

Keywords

Allometry, Scaling law, Animal vocalization, Phonation, Falsetto voice, Self-excited oscillation, Internal flow, Flow-structure interaction, Strouhal number, Dimensionless number

Paper information

Shinji DEGUCHI, “A possible common physical principle that underlies animal vocalization: theoretical considerations with an unsteady airflow-structure interaction model”, Journal of Biomechanical Science and Engineering, Vol.11, No.4 (2016), p.16-00414. doi:10.1299/jbse.16-00414. Final Version Released on December 16, 2016, Advance Publication Released on November 24, 2016.