Tension- and Adhesion-Regulated Retraction of Injured Axons.

Damage-induced retraction of axons during traumatic brain injury is believed to play a key role in the disintegration of the neural network and to eventually lead to severe symptoms such as permanent memory loss and emotional disturbances. However, fundamental questions such as how axon retraction progresses and what physical factors govern this process still remain unclear. Here, we report a combined experimental and modeling study to address these questions. Specifically, a sharp atomic force microscope probe was used to transect axons and trigger their retraction in a precisely controlled manner. Interestingly, we showed that the retracting motion of a well-developed axon can be arrested by strong cell-substrate attachment. However, axon retraction was found to be retriggered if a second transection was conducted, albeit with a lower shrinking amplitude. Furthermore, disruption of the actin cytoskeleton or cell-substrate adhesion significantly altered the retracting dynamics of injured axons. Finally, a mathematical model was developed to explain the observed injury response of neural cells in which the retracting motion was assumed to be driven by the pre-tension in the axon and progress against neuron-substrate adhesion as well as the viscous resistance of the cell. Using realistic parameters, model predictions were found to be in good agreement with our observations under a variety of experimental conditions. By revealing the essential physics behind traumatic axon retraction, findings here could provide insights on the development of treatment strategies for axonal injury as well as its possible interplay with other neurodegenerative diseases.

Rapid Plastic Deformation of Cancer Cells Correlates with High Metastatic Potential.

Metastasis plays a crucial role in tumor development, however, lack of quantitative methods to characterize the capability of cells to undergo plastic deformations has hindered the understanding of this important process. Here, a microfluidic system capable of imposing precisely controlled cyclic deformation on cells and therefore probing their viscoelastic and plastic characteristics is developed. Interestingly, it is found that significant plastic strain can accumulate rapidly in highly invasive cancer cell lines and circulating tumor cells (CTCs) from late-stage lung cancer patients with a characteristic time of a few seconds. In constrast, very little irreversible deformation is observed in the less invasive cell lines and CTCs from early-stage lung cancer patients, highlighting the potential of using the plastic response of cells as a novel marker in future cancer study. Furthermore, author showed that the observed irreversible deformation should originate mainly from cytoskeleton damage, rather than plasticity of the cell nucleus.

A Novel Electroporation System for Living Cell Staining and Membrane Dynamics Interrogation.

A novel electroporation system was developed to introduce transient membrane pores to cells in a spatially and temporally controlled manner, allowing us to achieve fast electrotransfection and live cell staining as well as to systematically interrogate the dynamics of the cell membrane. Specifically, using this platform, we showed that both reversible and irreversible electroporation could be induced in the cell population, with nano-sized membrane pores in the former case being able to self-reseal in ~10 min. In addition, green fluorescent protein(GFP)-vinculin plasmid and 543 phalloidin have been delivered successively into fibroblast cells, which enables us to monitor the distinct roles of vinculin and F-actin in cell adhesion and migration as well as their possible interplay during these processes. Compared to conventional bulk electroporation and staining methods, the new system offers advantages such as low-voltage operation, cellular level manipulation and testing, fast and adjustable transfection/staining and real-time monitoring; the new system therefore could be useful in different biophysical studies in the future.

Interaction and fusion dynamics between cellular blebs.

Membrane blebbing, as a mechanism for cells to regulate their internal pressure and membrane tension, is believed to play important roles in processes such as cell migration, spreading and apoptosis. However, the fundamental question of how different blebs interact with each other during their life cycles remains largely unclear. Here, we report a combined theoretical and experimental investigation to examine how the growth and retraction of a cellular bleb are influenced by neighboring blebs as well as the fusion dynamics between them. Specifically, a boundary integral model was developed to describe the shape evolution of cell membrane during the blebbing/retracting process. We showed that a drop in the intracellular pressure will be induced by the formation of a bleb whose retraction then restores the pressure level. Consequently, the volume that a second bleb can reach was predicted to heavily depend on its initial weakened size and the time lag with respect to the first bleb, all in quantitative agreement with our experimental observations. In addition, it was found that as the strength of membrane-cortex adhesion increases, the possible coalescence of two neighboring blebs changes from smooth fusion to abrupt coalescence and eventually to no fusion at all. Phase diagrams summarizing the dependence of such transition on key physical factors, such as the intracellular pressure and bleb separation, were also obtained.