Tissue Mechanics

Cells define and largely form their surrounding tissues and, in return, receive biochemical and physical cues from them. We are working on resolving this interdependence by quantifying these tissue mechanical properties, correlating them with biological function, investigating their origin and ultimately controlling them.
It is fairly well established by now that most, if not all, cells respond to mechanical cues of their microenvironment (see Mechanosensing) [1]. We are interested in understanding how the mechanical properties of tissues in vivo influence cellular behaviour in essential processes such as development and in general under homeostatic and pathological conditions. Thus, we investigate the mechanical properties of different soft tissues, such as neural and adipose tissues, at cellular resolution. For this we conduct indentation measurements by atomic force microscopy (AFM) optimized for such soft tissue samples [2, 3]. We combine these tissue mechanical measurements with a correlation of biological function during development, aging and pathology. For example, we have been working on the notion that neuronal growth is guided by mechanical cues. Mapping the mechanical properties of brain tissue using AFM in vivo revealed that the growth of the optic tract in Xenopus laevis is indeed following mechanical patterns [4]. Adverse mechanical properties of brain tissue after demyelination, for example in multiple sclerosis, might also lead to insufficient remyelination because of the mechanosensitivity of oligodendrocyte precursor cells [5].
To fully demonstrate that the mechanical properties are causative for normal or aberrant cell biological function we need to be able to intentionally tune these properties. While the key components of tissue architecture — cell types, extracellular matrix, intercellular connectivity and specific structures such as the capillary network — are in general well known, we still do not know whether and how these individual components contribute to defining the mechanical properties of the entire tissue. Hence, we have started to correlate quantitative tissue mechanical maps with advanced morphological analysis and their constituent make-up. Controlling tissue mechanical properties might eventually lead to entirely new therapeutic options for treating currently incurable neurological disorders.

An important technological approach towards this goal is also the realistic mimicking of 3D tissue mechanics in vitro . This allows us to examine and influence cellular behavior in response to altering the mechanical properties while fully controlling other parameters such as biochemistry or the topology of the cellular surrounding.

 

  1. P. Moshayedi, L. F. da Costa, A. F. Christ, S. P. Lacour, J. Fawcett, J. Guck, and K. Franze, “Mechanosensitivity of astrocytes on optimized polyacrylamide gels analyzed by quantitative morphometry,” Journal of Physics: Condensed Matter, vol. 22, iss. 19, p. 194114, 2010. doi:10.1088/0953-8984/22/19/194114
    [BibTeX] [Abstract] [Download PDF]

    Cells are able to detect and respond to mechanical cues from their environment. Previous studies have investigated this mechanosensitivity on various cell types, including neural cells such as astrocytes. In this study, we have carefully optimized polyacrylamide gels, commonly used as compliant growth substrates, considering their homogeneity in surface topography, mechanical properties, and coating density, and identified several potential pitfalls for the purpose of mechanosensitivity studies. The resulting astrocyte response to growth on substrates with shear storage moduli of G ‘ = 100 Pa and G ‘ = 10 kPa was then evaluated as a function of coating density of poly-D-lysine using quantitative morphometric analysis. Astrocytes cultured on stiff substrates showed significantly increased perimeter, area, diameter, elongation, number of extremities and overall complexity if compared to those cultured on compliant substrates. A statistically significant difference in the overall morphological score was confirmed with an artificial intelligence-based shape analysis. The dependence of the cells’ morphology on PDL coating density seemed to be weak compared to the effect of the substrate stiffness and was slightly biphasic, with a maximum at10–100 µg ml − 1 PDL concentration. Our finding suggests that the compliance of the surrounding tissue in vivo may influence astrocyte morphology and behavior.

    @Article{Mosha,
    Title = {Mechanosensitivity of astrocytes on optimized polyacrylamide gels analyzed by quantitative morphometry},
    Author = {Pouria Moshayedi and Luciano da F Costa and Andreas F. Christ and Stephanie P Lacour and James Fawcett and J.
    Guck and Kristian Franze},
    Journal = {{Journal of Physics: Condensed Matter}},
    Year = {2010},
    Number = {19},
    Pages = {194114},
    Volume = {22},
    Abstract = {Cells are able to detect and respond to mechanical cues from their environment. Previous studies have investigated this mechanosensitivity on various cell types, including neural cells such as astrocytes. In this study, we have carefully optimized polyacrylamide gels, commonly used as compliant growth substrates, considering their homogeneity in surface topography, mechanical properties, and coating density, and identified several potential pitfalls for the purpose of mechanosensitivity studies. The resulting astrocyte response to growth on substrates with shear storage moduli of G ' = 100 Pa and G ' = 10 kPa was then evaluated as a function of coating density of poly-D-lysine using quantitative morphometric analysis. Astrocytes cultured on stiff substrates showed significantly increased perimeter, area, diameter, elongation, number of extremities and overall complexity if compared to those cultured on compliant substrates. A statistically significant difference in the overall morphological score was confirmed with an artificial intelligence-based shape analysis. The dependence of the cells' morphology on PDL coating density seemed to be weak compared to the effect of the substrate stiffness and was slightly biphasic, with a maximum at10–100 µg ml − 1 PDL concentration. Our finding suggests that the compliance of the surrounding tissue in vivo may influence astrocyte morphology and behavior.},
    Doi = {10.1088/0953-8984/22/19/194114},
    Url = {http://iopscience.iop.org/article/10.1088/0953-8984/22/19/194114/pdf}
    }

  2. A. F. Christ, K. Franze, H. Gautier, P. Moshayedi, J. Fawcett, R. J. M. Franklin, R. T. Karadottir, and J. Guck, “Mechanical difference between white and gray matter in the rat cerebellum measured by scanning force microscopy,” Journal of Biomechanics, vol. 43, iss. 15, pp. 2986-2992, 2010. doi:10.1016/j.jbiomech.2010.07.002
    [BibTeX] [Abstract] [Download PDF]

    The mechanical properties of tissues are increasingly recognized as important cues for cell physiology and pathology. Nevertheless, there is a sparsity of quantitative, high-resolution data on mechanical properties of specific tissues. This is especially true for the central nervous system (CNS), which poses particular difficulties in terms of preparation and measurement. We have prepared thin slices of brain tissue suited for indentation measurements on the micrometer scale in a near-native state. Using a scanning force microscope with a spherical indenter of radius ∼20 μm we have mapped the effective elastic modulus of rat cerebellum with a spatial resolution of 100 μm. We found significant differences between white and gray matter, having effective elastic moduli of K=294±74 and 454±53 Pa, respectively, at 3 μm indentation depth (ng=245; nw=150 in four animals, p<0.05; errors are SD). In contrast to many other measurements on larger length scales, our results were constant for indentation depths of 2–4 μm indicating a regime of linear effective elastic modulus. These data, assessed with a direct mechanical measurement, provide reliable high-resolution information and serve as a quantitative basis for further neuromechanical investigations on the mechanical properties of developing, adult and damaged CNS tissue.

    @Article{Christ20102986,
    Title = {Mechanical difference between white and gray matter in the rat cerebellum measured by scanning force microscopy },
    Author = {Andreas F. Christ and Kristian Franze and Helene Gautier and Pouria Moshayedi and James Fawcett and Robin J.M. Franklin and Ragnhildur T. Karadottir and Jochen Guck},
    Journal = {{Journal of Biomechanics}},
    Year = {2010},
    Number = {15},
    Pages = {2986 - 2992},
    Volume = {43},
    Abstract = {The mechanical properties of tissues are increasingly recognized as important cues for cell physiology and pathology. Nevertheless, there is a sparsity of quantitative, high-resolution data on mechanical properties of specific tissues. This is especially true for the central nervous system (CNS), which poses particular difficulties in terms of preparation and measurement. We have prepared thin slices of brain tissue suited for indentation measurements on the micrometer scale in a near-native state. Using a scanning force microscope with a spherical indenter of radius ∼20 μm we have mapped the effective elastic modulus of rat cerebellum with a spatial resolution of 100 μm. We found significant differences between white and gray matter, having effective elastic moduli of K=294±74 and 454±53 Pa, respectively, at 3 μm indentation depth (ng=245; nw=150 in four animals, p<0.05; errors are SD). In contrast to many other measurements on larger length scales, our results were constant for indentation depths of 2–4 μm indicating a regime of linear effective elastic modulus. These data, assessed with a direct mechanical measurement, provide reliable high-resolution information and serve as a quantitative basis for further neuromechanical investigations on the mechanical properties of developing, adult and damaged CNS tissue. },
    Doi = {10.1016/j.jbiomech.2010.07.002},
    ISSN = {0021-9290},
    Keywords = {Atomic force microscopy (AFM)},
    Url = {http://www.sciencedirect.com/science/article/pii/S0021929010003660/pdfft?md5=a56195795ac714ff42c1f750fd85ad1d&pid=1-s2.0-S0021929010003660-main.pdf}
    }

  3. K. Franze, M. Francke, K. Günter, A. F. Christ, N. Körber, A. Reichenbach, and J. Guck, “Spatial mapping of the mechanical properties of the living retina using scanning force microscopy,” Soft matter, vol. 7, iss. 7, pp. 3147-3154, 2011. doi:doi:10.1039/c0sm01017k
    [BibTeX]

    @Article{Franze2011,
    Title = {Spatial mapping of the mechanical properties of the living retina using scanning force microscopy},
    Author = {Franze, Kristian and Francke, Mike and G{\"u}nter, Katrin and Christ, Andreas F and K{\"o}rber, Nicole and Reichenbach, Andreas and Guck, Jochen},
    Journal = {Soft Matter},
    Year = {2011},
    Number = {7},
    Pages = {3147--3154},
    Volume = {7},
    Doi = {doi:10.1039/c0sm01017k},
    Publisher = {{Royal Society of Chemistry}}
    }

  4. D. E. Koser, A. J. Thompson, S. K. Foster, A. Dwivedy, E. K. Pillai, G. K. Sheridan, H. Svoboda, M. Viana, L. F. da Costa, J. Guck, C. E. Holt, and K. Franze, “Mechanosensing is critical for axon growth in the developing brain,” Nature Neuroscience, vol. 19, iss. 12, pp. 1592-1598, 2016. doi:10.1038/nn.4394
    [BibTeX] [Abstract]

    During nervous system development, neurons extend axons along well-defined pathways. The current understanding of axon pathfinding is based mainly on chemical signaling. However, growing neurons interact not only chemically but also mechanically with their environment. Here we identify mechanical signals as important regulators of axon pathfinding. In vitro, substrate stiffness determined growth patterns of Xenopus retinal ganglion cell axons. In vivo atomic force microscopy revealed a noticeable pattern of stiffness gradients in the embryonic brain. Retinal ganglion cell axons grew toward softer tissue, which was reproduced in vitro in the absence of chemical gradients. To test the importance of mechanical signals for axon growth in vivo, we altered brain stiffness, blocked mechanotransduction pharmacologically and knocked down the mechanosensitive ion channel piezo1. All treatments resulted in aberrant axonal growth and pathfinding errors, suggesting that local tissue stiffness, read out by mechanosensitive ion channels, is critically involved in instructing neuronal growth in vivo.

    @Article{Koser2016,
    Title = {{Mechanosensing is critical for axon growth in the developing brain}},
    Author = {Koser, David E and Thompson, Amelia J and Foster, Sarah K and Dwivedy, Asha and Pillai, Eva K and Sheridan, Graham K and Svoboda, Hanno and Viana, Matheus and Costa, Luciano da F and Guck, Jochen and Holt, Christine E and Franze, Kristian},
    Journal = {{Nature Neuroscience}},
    Year = {2016},
    Month = {sep},
    Number = {12},
    Pages = {1592--1598},
    Volume = {19},
    Abstract = {During nervous system development, neurons extend axons along well-defined pathways. The current understanding of axon pathfinding is based mainly on chemical signaling. However, growing neurons interact not only chemically but also mechanically with their environment. Here we identify mechanical signals as important regulators of axon pathfinding. In vitro, substrate stiffness determined growth patterns of Xenopus retinal ganglion cell axons. In vivo atomic force microscopy revealed a noticeable pattern of stiffness gradients in the embryonic brain. Retinal ganglion cell axons grew toward softer tissue, which was reproduced in vitro in the absence of chemical gradients. To test the importance of mechanical signals for axon growth in vivo, we altered brain stiffness, blocked mechanotransduction pharmacologically and knocked down the mechanosensitive ion channel piezo1. All treatments resulted in aberrant axonal growth and pathfinding errors, suggesting that local tissue stiffness, read out by mechanosensitive ion channels, is critically involved in instructing neuronal growth in vivo.},
    Doi = {10.1038/nn.4394},
    File = {:C$\backslash$:/Users/Joan Carles Escolano/Desktop/PhD Dresden/Literature/Lab Recommendations/Mechanosensing is critical for axon growth in the developing brain (Nat Neur 2016).pdf:pdf},
    ISSN = {1097-6256},
    Owner = {paul},
    Publisher = {Nature Research},
    Timestamp = {2017.01.25}
    }

  5. A. Jagielska, A. L. Norman, G. Whyte, K. V. J. Vliet, J. Guck, and R. J. M. Franklin, “Mechanical Environment Modulates Biological Properties of Oligodendrocyte Progenitor Cells,” Stem Cells and Development, vol. 21, iss. 16, 2012. doi:10.1089/scd.2012.0189
    [BibTeX] [Abstract]

    Myelination and its regenerative counterpart remyelination represent one of the most complex cell–cell interactions in the central nervous system (CNS). The biochemical regulation of axon myelination via the proliferation, migration, and differentiation of oligodendrocyte progenitor cells (OPCs) has been characterized extensively. However, most biochemical analysis has been conducted in vitro on OPCs adhered to substrata of stiffness that is orders of magnitude greater than that of the in vivo CNS environment. Little is known of how variation in mechanical properties over the physiological range affects OPC biology. Here, we show that OPCs are mechanosensitive. Cell survival, proliferation, migration, and differentiation capacity in vitro depend on the mechanical stiffness of polymer hydrogel substrata. Most of these properties are optimal at the intermediate values of CNS tissue stiffness. Moreover, many of these properties measured for cells on gels of optimal stiffness differed significantly from those measured on glass or polystyrene. The dependence of OPC differentiation on the mechanical properties of the extracellular environment provides motivation to revisit results obtained on nonphysiological, rigid surfaces. We also find that OPCs stiffen upon differentiation, but that they do not change their compliance in response to substratum stiffness, which is similar to embryonic stem cells, but different from adult stem cells. These results form the basis for further investigations into the mechanobiology of cell function in the CNS and may specifically shed new light on the failure of remyelination in chronic demyelinating diseases such as multiple sclerosis.

    @Article{Jagielska2012,
    Title = {{Mechanical Environment Modulates Biological Properties of Oligodendrocyte Progenitor Cells}},
    Author = {Jagielska, Anna and Norman, Adele L and Whyte, Graeme and Vliet, Krystyn J Van and Guck, Jochen and Franklin, Robin J M},
    Journal = {{Stem Cells and Development}},
    Year = {2012},
    Number = {16},
    Volume = {21},
    Abstract = {Myelination and its regenerative counterpart remyelination represent one of the most complex cell–cell interactions in the central nervous system (CNS). The biochemical regulation of axon myelination via the proliferation, migration, and differentiation of oligodendrocyte progenitor cells (OPCs) has been characterized extensively. However, most biochemical analysis has been conducted in vitro on OPCs adhered to substrata of stiffness that is orders of magnitude greater than that of the in vivo CNS environment. Little is known of how variation in mechanical properties over the physiological range affects OPC biology. Here, we show that OPCs are mechanosensitive. Cell survival, proliferation, migration, and differentiation capacity in vitro depend on the mechanical stiffness of polymer hydrogel substrata. Most of these properties are optimal at the intermediate values of CNS tissue stiffness. Moreover, many of these properties measured for cells on gels of optimal stiffness differed significantly from those measured on glass or polystyrene. The dependence of OPC differentiation on the mechanical properties of the extracellular environment provides motivation to revisit results obtained on nonphysiological, rigid surfaces. We also find that OPCs stiffen upon differentiation, but that they do not change their compliance in response to substratum stiffness, which is similar to embryonic stem cells, but different from adult stem cells. These results form the basis for further investigations into the mechanobiology of cell function in the CNS and may specifically shed new light on the failure of remyelination in chronic demyelinating diseases such as multiple sclerosis.},
    Doi = {10.1089/scd.2012.0189},
    File = {:C$\backslash$:/Users/Joan Carles Escolano/Desktop/PhD Dresden/Literature/Lab Recommendations/Mechanical Environment Modulates Biological Properties of Oligodendrocyte Progenitor Cells(Stem Cells and Dev 2012).pdf:pdf},
    Owner = {paul},
    Timestamp = {2017.01.25}
    }