57


ALEXANDER A. SPECTOR, Ph.D.

Research Professor, Fellow of ASME





Cell (stem cell) mechanics and biophysics


Cell mechanical properties, force generation,, and adhesion are critical to cell behavior, such as migration, motility, growth, and division, under normal and pathological conditions. Cells are subjected to various mechanical stimuli from their microenvironment causing biochemical and electrical signals. Mechanical forces, strains, stiffness, and their gradients are important to morphogenesis. Stem cells commit (switch) to specific lineages depending on the mechanical properties/topography of the extracellular matrix and applied mechanical stimuli. The mechanical properties are considered now as biomarkers of the stage of a disease. Cancer cells have different mechanical properties and they are more motile (especially in metastatic stages) compared to normal cells. Changes in the pattern of local shear stresses acting on endothelial cells are linked to atherosclerotic plaques. The mechanical properties and shape of the cell nucleus change as a result of mutation of genes encoding specific proteins in the nuclear envelope associated with a wide range of diseases, including some forms of muscular dystrophy. We develop mathematical and computational models to analyze the fundamentals of cell mechanics and biophysics and interpret experimental data. These are several examples of our studies.


Modeling stem cell proliferation, self-renewal, and differentiation requires consideration of interrelated transcription, signaling, and environmental factors. Under physiological conditions, skeletal muscle regeneration relies on specialized stem cells called satellite cells. However, this mechanism fails under pathological conditions of muscular dystrophy, aging, and severe muscle trauma, and the use of other stem cells are needed. One promising approach is based on adipose-derived stem cells (ASCs) that are abundant and easily accessible in the patient's body. The outcome of ASC myogenesis can be significantly improved by the application of the mechanical factors (e.g. uniaxial strains) (P. Huri, A. Wang, A. Spector, and W. Grayson, Cellular and Molecular Bioengineering, 2014, 7:497-509)



Definition of major stages and environmental factors of ASC myogenesis


We developed a coordinated modeling-experimental approach to the analysis of ASC myogenesis, including a kinetic model of cell transition through several stages defined by the expression of myogenic factors observed in the experiment. Our approach helps in a better understanding and optimization of the mechanism of ASC myogenesis. (R. Deshpande, W. Grayson, and A. Spector, PLoS One, 2015, 10, e0137918)



Modeling results of the kinetics of ASC myogenesis vs. experimental data in the dynamic (10% strain) and static cases



Outer hair cells in mammalian cochleae are unique because they are capable of generating an active force of almost constant amplitude and phase up to tens of kHz in response to changes in the cell membrane electric potential. An electromechanical model to reproduce this cell's behavior in such a broad frequency range is challenging. We have proposed a model describing the major features of the outer hair cell within acoustic frequency range. The model derived from thermodynamic principles is in the form of integral relationships between the history of voltage and the resultants in cell's composite membrane as independent variables and the charge density and strains as dependent variables. This model generalizes piezolelectric and viscoelastic constitutive models. We show that the frequency-invariance of the cell active force is a result of interplay between the electrical filtering associated with the membrane protein prestin and power law viscoelasticity of the surrounding membrane (S. Roy, W. Brownell, and A. Spector, PloS One, 2012, 7, art. e37667).


Cell protrusions are deformations of cell surface that occur during biological processes such as formation of lamellipodia and filopodia in migrating cells. They can also be induced by inflammation in the form of attachments of the surface of leucocytes rolling over endothelial cell layer. A linear viscoelastic model was applied to the experimental data obtained from a combination of optical tweezers and fluorescent microscopy to extract the viscoelastic properties of the protruded cell membranes under normal conditions as well as under conditions of elevated and depleted membrane cholesterol. The obtained results can be important for a better understanding of the effects of membrane composition and membrane interaction with the cytoskeleton on the mechanics of cell migration and mechanotransduction (N. Hatibzadeh, A. Spector, W. Brownell, and B. Anvari, PLoS One, 2013, 8, art. e57147)


Modeling cochlear outer hair cells and understanding active hearing


Mammalian cochleae have special cells, called outer hair cells, which provide the amplification and sharp frequency selectivity of the ear. These cells do not regenerate and therefore their loss and damage cause hearing impairment.  The loss of outer hair cells is an important reason of hearing decline with aging. Acoustic trauma due to excessive noise results in damage of hair cells and causes hearing loss. The active properties of healthy outer hair cells can also be affected by some components of medications such as salycilate. Although human hair cells do not regenerate naturally it is believed now that they can be replaced by induced stem cells based on hair cells of lower species or other cells in the cochlea. Somatic motility of outer hair cells is critical to this cell's active force production up to tens of kHz. The protein prestin is distributed along the plasma membrane of the composite outer hair cell wall, and it is crucial to the outer hair cell active properties.


We use a modeling approach to describe the outer hair cell behavior at the interconnected molecular (prestin), cellular, and organ levels. We propose a piezoelectric-like model of the outer hair cell wall and extend it to cover cell's high-frequency performance (A. Spector, 1999; S. Roy, W. Brownell, and A. Spector, PloS One, 2012, 7, art. e37667). One of the long-standing questions to the mechanism of outer hair cell motility is the cell membrane filtering of the high-frequency receptor (driving) potentials. We show that the electromechanical coupling in the outer hair cell membrane associated with prestin and mechanosensitive channels can balance the membrane electrical filtering (A. Spector, W. Brownell, and A. Popel, Ann. Biomed Eng., 2003, 113:453-461. A. Spector, A. Popel, R.-A. Eatock, and W. Brownell, J. Acoust Soc. Am., 2005: 23:991-1002).  We also analyze the outer hair cell active force production in the cochlear environment (Z. Liao, S. Feng, A. Popel, W. Brownell, and A. Spector, J. Acoust. Soc. Am., 2007, 122:2215-2225). Prestin-associated charge transfer is an intrinsic part of outer hair cell electromotility, and the nature of this charge (internal vs. external to the protein and inside vs. through the membrane) is being actively debated. We analyze this phenomenon and take into account the amplitude and frequency of the applied electric field as well as the properties of the surrounding membrane (N. Nilsen, W. Brownell, S. Sun, and A. Spector, Biomech. & Model. In Mechanobiol, 2012, 11:107-118; S. Sun, B. Farrell, M. Chana, G. Oster, W. Brownell, and A. Spector. J. Theoret. Biol., 2009, 260: 137-144).


Effects of the membrane mechanics on prestin-associated charge transfer in the native cochlear outer hair cells and cells transfected with prestin.



Mechanics (electromechanics)  of cellular membranes

 

Cellular membranes are critical to cell homeostasis and they contain membrane proteins sensing the mechanical stimuli. The membrane mechanical properties reflect membrane lipid composition and membrane interaction with the cell cytoskeleton. Electric charges of different nature are intrinsic to biological membrane and they result in coupling between membrane electrical (charges, potential) and mechanical (strains, bending) properties. We develop biophysical and computational models of cell membranes to better understand membrane electromechanics and interpret experiments. These are examples of our studies of cellular membranes.

It was recently discovered that the mechanotransduction channel, which is a critical part of mechanotransduction machinery in hair cells of organs of hearing and balance, is located in the tip membrane at the bottom of the tip link connecting the neighboring shorter and taller stereocilia. The force that governs the opening and closing of this channel is transmitted via the tip link and the concept of a phenomenological gating spring has been important to understanding mechanotransduction in the hair cell stereocilia. We have developed a computational model of the membrane environment around the channel and have shown that the tip membrane can serve as the gating spring, by itself or in a combination with a tether connecting the channel to the stereocilia actin core. We have also studied the distribution of local forces around the channel that are generated in the highly-curved membrane by the tip link (R. Powers, S. Roy, E. Atilgan, W. Brownell, S. Sun, P. Gillespie, and A. Spector, Biophys. J., 2012, 102:291-210).



Computational modeling of the membrane forces (resultatnts) acting on the mechanotransduction channel in the tip membrane of shorter stereocilia.


Membrane tethers can occur naturally during cell migration or intercellular transport, and they are also used as a means to probe the membrane mechanical and electromechanical properties. Under thin tether conditions, the membrane curvature is high and it plays a significant role in the membrane mechanics. We have developed a computational model of describe the membrane in the tether pulling experiment. The membrane has three characteristic regions, the body of the tether, zone of the membrane-cytoskeleton attachment, and a transition area between the two. We use curvature-related terms in the membrane resultants and obtain the force, tension, and energy mode distribution throughout the considered three-region area of the membrane (C. Lau, W. Brownell, and A. Spector, J. Biomech., 2012, 45:1326-1331.; K. Schumacher, A. Popel, B. Anvari, W. Brownell, and A. Spector, Phys. Review. E., 2009, 80, art 041905).



Sketch of the tether pulling experiment with force application to a bead adhered to the cell membrane surface and an axisymmetric model of the membrane with four regions considered.