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Matrix viscoplasticity and its shielding by active mechanics in microtissue models: experiments and mathematical modeling

View Article: PubMed Central - PubMed

ABSTRACT

The biomechanical behavior of tissues under mechanical stimulation is critically important to physiological function. We report a combined experimental and modeling study of bioengineered 3D smooth muscle microtissues that reveals a previously unappreciated interaction between active cell mechanics and the viscoplastic properties of the extracellular matrix. The microtissues’ response to stretch/unstretch actuations, as probed by microcantilever force sensors, was dominated by cellular actomyosin dynamics. However, cell lysis revealed a viscoplastic response of the underlying model collagen/fibrin matrix. A model coupling Hill-type actomyosin dynamics with a plastic perfectly viscoplastic description of the matrix quantitatively accounts for the microtissue dynamics, including notably the cells’ shielding of the matrix plasticity. Stretch measurements of single cells confirmed the active cell dynamics, and were well described by a single-cell version of our model. These results reveal the need for new focus on matrix plasticity and its interactions with active cell mechanics in describing tissue dynamics.

No MeSH data available.


Related in: MedlinePlus

Schematic of matrix mechanics emerging from current studies.(a) Stress-strain space diagram for the matrix in the microtissues with schematics showing the relative deflections of the micropillars. Red indicates that the magnetic field is active. The moment of cell seeding is taken to be the zero stress and zero strain condition (Point A). As the microtissue matures, cell contraction compresses the matrix (Point B). Mechanical loading and unloading drives the matrix between points B and C, causing reversible stretching (lower right Inset). Treating microtissues with Triton X-100 lyses the cells placing the matrix under tension due to the outward stress exerted by the micropillars with no counterbalancing active cellular contraction (Point D). Applying the loading profile to Triton-treated microtissues drives the matrix to point E. However, they never fully recover to point D as the tension yield stress  is exceeded (upper right Inset). (b–e) Illustrations of the state of the ECM during these processes, highlighting the shielding effects of the cells. (b) SMCs (black ovals) are dispersed throughout the cross-linked (orange circles) collagen ECM (dashed green lines) and bind with it at focal adhesion sites. The total magnitude of the cellular active contractility Fcell is represented by the red arrows. Contractile forces are directed towards the cells’ centers, and the SMCs act as nodes of contractility for the ECM network. Blue bar represents cell-cell mechanical linkages. (c) As the tissue is stretched through the application of an external force, tissue stress σ increases as does the levels of cellular force generation. Due to the active contractility of the cells, the collagen network is shielded from plastic deformation caused by this stress increase, and mainly deforms elastically. (d) Permeabilizing the cell membrane and lysing the cells through Triton X-100 treatment places the microtissue under tension due to the stress produced by the deflected micropillars (). (e) Applying an additional external stress σ causes irreversible changes in the ECM microstructure through fiber alignment and changes in crosslinking, leading to plastic deformation.
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f5: Schematic of matrix mechanics emerging from current studies.(a) Stress-strain space diagram for the matrix in the microtissues with schematics showing the relative deflections of the micropillars. Red indicates that the magnetic field is active. The moment of cell seeding is taken to be the zero stress and zero strain condition (Point A). As the microtissue matures, cell contraction compresses the matrix (Point B). Mechanical loading and unloading drives the matrix between points B and C, causing reversible stretching (lower right Inset). Treating microtissues with Triton X-100 lyses the cells placing the matrix under tension due to the outward stress exerted by the micropillars with no counterbalancing active cellular contraction (Point D). Applying the loading profile to Triton-treated microtissues drives the matrix to point E. However, they never fully recover to point D as the tension yield stress is exceeded (upper right Inset). (b–e) Illustrations of the state of the ECM during these processes, highlighting the shielding effects of the cells. (b) SMCs (black ovals) are dispersed throughout the cross-linked (orange circles) collagen ECM (dashed green lines) and bind with it at focal adhesion sites. The total magnitude of the cellular active contractility Fcell is represented by the red arrows. Contractile forces are directed towards the cells’ centers, and the SMCs act as nodes of contractility for the ECM network. Blue bar represents cell-cell mechanical linkages. (c) As the tissue is stretched through the application of an external force, tissue stress σ increases as does the levels of cellular force generation. Due to the active contractility of the cells, the collagen network is shielded from plastic deformation caused by this stress increase, and mainly deforms elastically. (d) Permeabilizing the cell membrane and lysing the cells through Triton X-100 treatment places the microtissue under tension due to the stress produced by the deflected micropillars (). (e) Applying an additional external stress σ causes irreversible changes in the ECM microstructure through fiber alignment and changes in crosslinking, leading to plastic deformation.

Mentions: Due to the shielding of the ECM’s plastic behavior by the cells, accessing the mechanical properties of the ECM required inhibition of the active cellular contribution to the tissues. We achieved this through Triton X-100-induced lysis of the cells. The picture of the matrix mechanics that emerges from these studies is illustrated Fig. 5a, which shows a stress-strain phase space diagram of the matrix component of the microtissues. Here, the zero stress and zero strain condition is taken to be the moment immediately after seeding the cells and matrix in the microwells (Point A). The distributed active contractile forces produced by the cells dispersed in the matrix placed the matrix in the constructs under compression (Point B) as the matrix compacted from its original (seeding) density as the microtissues formed. We note that the stress field inside the microtissues will have some heterogeneity, and despite the overall compression, local regions between cells may even be in tension as they transmit traction forces between cells18. This is captured by the element Es in our model. However, once the active cellular contractility was eliminated via Triton treatment, it became clear that the ECM had in fact yielded under the compressive stress during the tissue formation process. This is evident since, after Triton treatment (Point D), the pillars supporting the tissues remained inwardly deflected, indicating that the microtissues retained residual elasticity that did not depend on the cells. This irreversible change in the matrix during the microtissue formation may be attributed to collagen compaction and subsequent crosslinking as seen in fibroblast constructs1128, which is consistent with the recently observed time-dependent inelastic behavior of collagen networks29. The compacted, crosslinked state of the as-grown tissues is sketched in Fig. 5b. The mechanical loading and unloading of the tissues during stretch, unstretch, and subsequent active recovery (Path B −>C −>B in Fig. 5a), caused essentially reversible changes in the ECM. This is illustrated pictorially in Fig. 5c, and in the lower inset to Fig. 5a, which shows the matrix component to the total stress-strain response of Tissue C2, as determined from the fit shown in Supplementary Fig. 3. The approximately 10 times smaller scale for the changes in the matrix stress during loading compared to that of the overall stress changes highlights the minimal contribution of the stiffness of the ECM when loaded while under compression.


Matrix viscoplasticity and its shielding by active mechanics in microtissue models: experiments and mathematical modeling
Schematic of matrix mechanics emerging from current studies.(a) Stress-strain space diagram for the matrix in the microtissues with schematics showing the relative deflections of the micropillars. Red indicates that the magnetic field is active. The moment of cell seeding is taken to be the zero stress and zero strain condition (Point A). As the microtissue matures, cell contraction compresses the matrix (Point B). Mechanical loading and unloading drives the matrix between points B and C, causing reversible stretching (lower right Inset). Treating microtissues with Triton X-100 lyses the cells placing the matrix under tension due to the outward stress exerted by the micropillars with no counterbalancing active cellular contraction (Point D). Applying the loading profile to Triton-treated microtissues drives the matrix to point E. However, they never fully recover to point D as the tension yield stress  is exceeded (upper right Inset). (b–e) Illustrations of the state of the ECM during these processes, highlighting the shielding effects of the cells. (b) SMCs (black ovals) are dispersed throughout the cross-linked (orange circles) collagen ECM (dashed green lines) and bind with it at focal adhesion sites. The total magnitude of the cellular active contractility Fcell is represented by the red arrows. Contractile forces are directed towards the cells’ centers, and the SMCs act as nodes of contractility for the ECM network. Blue bar represents cell-cell mechanical linkages. (c) As the tissue is stretched through the application of an external force, tissue stress σ increases as does the levels of cellular force generation. Due to the active contractility of the cells, the collagen network is shielded from plastic deformation caused by this stress increase, and mainly deforms elastically. (d) Permeabilizing the cell membrane and lysing the cells through Triton X-100 treatment places the microtissue under tension due to the stress produced by the deflected micropillars (). (e) Applying an additional external stress σ causes irreversible changes in the ECM microstructure through fiber alignment and changes in crosslinking, leading to plastic deformation.
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Related In: Results  -  Collection

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f5: Schematic of matrix mechanics emerging from current studies.(a) Stress-strain space diagram for the matrix in the microtissues with schematics showing the relative deflections of the micropillars. Red indicates that the magnetic field is active. The moment of cell seeding is taken to be the zero stress and zero strain condition (Point A). As the microtissue matures, cell contraction compresses the matrix (Point B). Mechanical loading and unloading drives the matrix between points B and C, causing reversible stretching (lower right Inset). Treating microtissues with Triton X-100 lyses the cells placing the matrix under tension due to the outward stress exerted by the micropillars with no counterbalancing active cellular contraction (Point D). Applying the loading profile to Triton-treated microtissues drives the matrix to point E. However, they never fully recover to point D as the tension yield stress is exceeded (upper right Inset). (b–e) Illustrations of the state of the ECM during these processes, highlighting the shielding effects of the cells. (b) SMCs (black ovals) are dispersed throughout the cross-linked (orange circles) collagen ECM (dashed green lines) and bind with it at focal adhesion sites. The total magnitude of the cellular active contractility Fcell is represented by the red arrows. Contractile forces are directed towards the cells’ centers, and the SMCs act as nodes of contractility for the ECM network. Blue bar represents cell-cell mechanical linkages. (c) As the tissue is stretched through the application of an external force, tissue stress σ increases as does the levels of cellular force generation. Due to the active contractility of the cells, the collagen network is shielded from plastic deformation caused by this stress increase, and mainly deforms elastically. (d) Permeabilizing the cell membrane and lysing the cells through Triton X-100 treatment places the microtissue under tension due to the stress produced by the deflected micropillars (). (e) Applying an additional external stress σ causes irreversible changes in the ECM microstructure through fiber alignment and changes in crosslinking, leading to plastic deformation.
Mentions: Due to the shielding of the ECM’s plastic behavior by the cells, accessing the mechanical properties of the ECM required inhibition of the active cellular contribution to the tissues. We achieved this through Triton X-100-induced lysis of the cells. The picture of the matrix mechanics that emerges from these studies is illustrated Fig. 5a, which shows a stress-strain phase space diagram of the matrix component of the microtissues. Here, the zero stress and zero strain condition is taken to be the moment immediately after seeding the cells and matrix in the microwells (Point A). The distributed active contractile forces produced by the cells dispersed in the matrix placed the matrix in the constructs under compression (Point B) as the matrix compacted from its original (seeding) density as the microtissues formed. We note that the stress field inside the microtissues will have some heterogeneity, and despite the overall compression, local regions between cells may even be in tension as they transmit traction forces between cells18. This is captured by the element Es in our model. However, once the active cellular contractility was eliminated via Triton treatment, it became clear that the ECM had in fact yielded under the compressive stress during the tissue formation process. This is evident since, after Triton treatment (Point D), the pillars supporting the tissues remained inwardly deflected, indicating that the microtissues retained residual elasticity that did not depend on the cells. This irreversible change in the matrix during the microtissue formation may be attributed to collagen compaction and subsequent crosslinking as seen in fibroblast constructs1128, which is consistent with the recently observed time-dependent inelastic behavior of collagen networks29. The compacted, crosslinked state of the as-grown tissues is sketched in Fig. 5b. The mechanical loading and unloading of the tissues during stretch, unstretch, and subsequent active recovery (Path B −>C −>B in Fig. 5a), caused essentially reversible changes in the ECM. This is illustrated pictorially in Fig. 5c, and in the lower inset to Fig. 5a, which shows the matrix component to the total stress-strain response of Tissue C2, as determined from the fit shown in Supplementary Fig. 3. The approximately 10 times smaller scale for the changes in the matrix stress during loading compared to that of the overall stress changes highlights the minimal contribution of the stiffness of the ECM when loaded while under compression.

View Article: PubMed Central - PubMed

ABSTRACT

The biomechanical behavior of tissues under mechanical stimulation is critically important to physiological function. We report a combined experimental and modeling study of bioengineered 3D smooth muscle microtissues that reveals a previously unappreciated interaction between active cell mechanics and the viscoplastic properties of the extracellular matrix. The microtissues’ response to stretch/unstretch actuations, as probed by microcantilever force sensors, was dominated by cellular actomyosin dynamics. However, cell lysis revealed a viscoplastic response of the underlying model collagen/fibrin matrix. A model coupling Hill-type actomyosin dynamics with a plastic perfectly viscoplastic description of the matrix quantitatively accounts for the microtissue dynamics, including notably the cells’ shielding of the matrix plasticity. Stretch measurements of single cells confirmed the active cell dynamics, and were well described by a single-cell version of our model. These results reveal the need for new focus on matrix plasticity and its interactions with active cell mechanics in describing tissue dynamics.

No MeSH data available.


Related in: MedlinePlus