Previous research

Microrheology

 In order to understand how activity can control the mechanical properties of life, it is necessary to measure both the stiffness and the degree of (non-equilibrium) activity. We carry this out with state-of-the-art "simultaneous active/passive microrheology".

 

 

 

AFM-Microrheology system

 

Nonequilibrium mechanics of life

 

 Any style of life, from cells to tissues to whole organisms, is active-driven by spontaneously generated forces. Within cells various organelles, vesicles and lipids are found to be actively fluctuating when observed by high contrast imaging techniques (movie 1). Cells themselves also migrate (movie 2). Studying the strong nonequilibrium state in cells is critical for understanding the mechanism of various cellular processes such as cell migration (movie 2), division, intracellular material production and transportation.
 Recently, it has been realized that cells regulate their own properties by their nonequilibrium activity. How much they are “alive” determines their physical characters such as mechanical property without changing any chemical constituents. We study this enigmatic mechanism by creating simple model systems composed of, for instance, cytoskeleton structures, lipid membranes and DNA.

movie1: Nonequilibrium fluctuations in a cultured cell
(DIC image)
movie2: Cell migration

 

  Inside cells, there exists a network called the cytoskeleton composed of semi-flexible polymers such as actin, microtubules, and intermediate filaments. The cytoskeleton provides mechanical strength to cells and facilitates dyniamics processes by generating forces. All these are analogous to the mechanical design of higher organisms.

  Functions of the cytoskeleton (actin etc.)
+motor proteins (myosin etc.)

Provides mehcanical strength

 

Facilitats dynamic processes:
cell division, migration...

   

Nonequilibrium fluctuation of actin/myosin

 As our body hardness and shape is determined by their associated organsm (miscle), cell structure is determined by tissue protein called "cytoskeleton". In our body, muscle associated with skeleton generate forces. On the othe hand, nanometer sized molecular machine (motor protein) generate forces with cytoskeleton in cytoplasm. Thus, cells become a out-of-equilibrium state. Real life is too complicated for physicists to work with. We therefore extract the most fundamental components for intracellular force generations and reconstitute them to produce force generating gels in vitro. We call these gels "active gels". Active gels look as if they are alive as can be seen below (movie3, movie4).

Non -equilibrium fluctuations in non-closslinked actin/myosin gels

movie3:nonequilibrium fluctuations generated by actin/myosin movie4: Super precipitation caused by force generation

 

Force generation mechanism on actin/myosin system

 

Non-equilibrium fluctuations in closslinked actin/myosin gel

Superprecipitation because of ATP depletion

 Coss-linked gels can support motor-generated stress (left above). We analyze the motion of probe particles dispersed in the gel. Without motor-generated stress, it is hard to observe such large fluctuations.

 

Mechano sensing

 It hurts for humans to get punched (Fig . 1), and so it does for cells, which is called "mechano-sensing". Here fibronectin-coated colloiidal particles were adhered to a cell (movie 5), and the cell was punched with an optical tweezers(left below). We observed release of a second messenger,NO in response to the punches. (Fig. 2). Cells were labelled with fluorescent NO indicator.

Fig. 1: Hurt for human to get punch  
movie 5: Add force on outside of the cell Fig. 2: Fluorescence of indicator linked to NO