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Microglia mechanics: immune activation alters traction forces and durotaxis.

Bollmann L, Koser DE, Shahapure R, Gautier HO, Holzapfel GA, Scarcelli G, Gather MC, Ulbricht E, Franze K - Front Cell Neurosci (2015)

Bottom Line: Microglial cells are key players in the primary immune response of the central nervous system.They are highly active and motile cells that chemically and mechanically interact with their environment.Our results demonstrate that microglia are susceptible to mechanical signals, which could be important during central nervous system development and pathologies.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Development and Neuroscience, University of Cambridge Cambridge, UK ; Faculty of Computer Science and Biomedical Engineering, Institute of Biomechanics, Graz University of Technology Graz, Austria.

ABSTRACT
Microglial cells are key players in the primary immune response of the central nervous system. They are highly active and motile cells that chemically and mechanically interact with their environment. While the impact of chemical signaling on microglia function has been studied in much detail, the current understanding of mechanical signaling is very limited. When cultured on compliant substrates, primary microglial cells adapted their spread area, morphology, and actin cytoskeleton to the stiffness of their environment. Traction force microscopy revealed that forces exerted by microglia increase with substrate stiffness until reaching a plateau at a shear modulus of ~5 kPa. When cultured on substrates incorporating stiffness gradients, microglia preferentially migrated toward stiffer regions, a process termed durotaxis. Lipopolysaccharide-induced immune-activation of microglia led to changes in traction forces, increased migration velocities and an amplification of durotaxis. We finally developed a mathematical model connecting traction forces with the durotactic behavior of migrating microglial cells. Our results demonstrate that microglia are susceptible to mechanical signals, which could be important during central nervous system development and pathologies. Stiffness gradients in tissue surrounding neural implants such as electrodes, for example, could mechanically attract microglial cells, thus facilitating foreign body reactions detrimental to electrode functioning.

No MeSH data available.


Related in: MedlinePlus

Durotaxis of microglial cells. (A) Distribution of microglial cell positions at the end of an experiment. Cells were cultured on compliant substrates (~5 kPa) incorporating a stiffness gradient of ~8 Pa/μm and preferentially migrated toward the stiffer side of the substrate (N = 38). (B) After activation with LPS, microglia durotaxis was significantly enhanced (N = 22). Rose plots were obtained from binned endpoints (bin size: of 60°, with 20° overlap). (C) Schematic plot of microglia migration. Start and end positions are marked with open triangles; black dots indicate cell positions recorded during time-lapse imaging. The blue curve shows the actual migration path (“contour”), the red line indicates the Euclidean distance between start and end position. (D) The directness D was similar for control and LPS-treated cells (p > 0.7, Mann-Whitney U-test). (E,G) Contour velocity vc and Euclidean velocity ve significantly increased after application of LPS (p < 0.05, Mann-Whitney U-test). (F) Stalling phases were highly reduced after LPS treatment (p < 10−4, Mann-Whitney U-test). (H,I) While the component of the Euclidean velocity perpendicular to the mechanical gradient (x-component) was similar for control and LPS-treated cells (p > 0.2, Mann-Whitney U-test), the component toward the stiffer side of the gradient (y-component) was significantly increased after application of LPS (p < 0.05, Mann-Whitney U-test). *p < 0.05; ***p < 0.001.
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Figure 6: Durotaxis of microglial cells. (A) Distribution of microglial cell positions at the end of an experiment. Cells were cultured on compliant substrates (~5 kPa) incorporating a stiffness gradient of ~8 Pa/μm and preferentially migrated toward the stiffer side of the substrate (N = 38). (B) After activation with LPS, microglia durotaxis was significantly enhanced (N = 22). Rose plots were obtained from binned endpoints (bin size: of 60°, with 20° overlap). (C) Schematic plot of microglia migration. Start and end positions are marked with open triangles; black dots indicate cell positions recorded during time-lapse imaging. The blue curve shows the actual migration path (“contour”), the red line indicates the Euclidean distance between start and end position. (D) The directness D was similar for control and LPS-treated cells (p > 0.7, Mann-Whitney U-test). (E,G) Contour velocity vc and Euclidean velocity ve significantly increased after application of LPS (p < 0.05, Mann-Whitney U-test). (F) Stalling phases were highly reduced after LPS treatment (p < 10−4, Mann-Whitney U-test). (H,I) While the component of the Euclidean velocity perpendicular to the mechanical gradient (x-component) was similar for control and LPS-treated cells (p > 0.2, Mann-Whitney U-test), the component toward the stiffer side of the gradient (y-component) was significantly increased after application of LPS (p < 0.05, Mann-Whitney U-test). *p < 0.05; ***p < 0.001.

Mentions: Cells cultured on these compliant substrates migrated comparatively straight; the median directness D (Equation 2), which is the ratio between the shortest linear distance between the start and endpoints of a path and the total travel distance, was 0.73. Furthermore, microglia stopped frequently, with a median stopping rate of 41% (i.e., no movement between 41% of consecutive frames, see Materials and Methods), and traveled with a median contour velocity vc (based on the contour length of the migration path; Equation 3) of 1.48 μm/min, and a median Euclidean velocity ve (based on a straight line between start and end position; Equation 4 and Figure 6C) of 0.82 μm/min (Figure 6). Endpoints of the migration paths showed a non-uniform circular distribution (p < 0.01, Rayleigh test) and were strongly biased toward the stiffer side of the gradient (Figure 6A), indicating that microglial cells showed durotaxis.


Microglia mechanics: immune activation alters traction forces and durotaxis.

Bollmann L, Koser DE, Shahapure R, Gautier HO, Holzapfel GA, Scarcelli G, Gather MC, Ulbricht E, Franze K - Front Cell Neurosci (2015)

Durotaxis of microglial cells. (A) Distribution of microglial cell positions at the end of an experiment. Cells were cultured on compliant substrates (~5 kPa) incorporating a stiffness gradient of ~8 Pa/μm and preferentially migrated toward the stiffer side of the substrate (N = 38). (B) After activation with LPS, microglia durotaxis was significantly enhanced (N = 22). Rose plots were obtained from binned endpoints (bin size: of 60°, with 20° overlap). (C) Schematic plot of microglia migration. Start and end positions are marked with open triangles; black dots indicate cell positions recorded during time-lapse imaging. The blue curve shows the actual migration path (“contour”), the red line indicates the Euclidean distance between start and end position. (D) The directness D was similar for control and LPS-treated cells (p > 0.7, Mann-Whitney U-test). (E,G) Contour velocity vc and Euclidean velocity ve significantly increased after application of LPS (p < 0.05, Mann-Whitney U-test). (F) Stalling phases were highly reduced after LPS treatment (p < 10−4, Mann-Whitney U-test). (H,I) While the component of the Euclidean velocity perpendicular to the mechanical gradient (x-component) was similar for control and LPS-treated cells (p > 0.2, Mann-Whitney U-test), the component toward the stiffer side of the gradient (y-component) was significantly increased after application of LPS (p < 0.05, Mann-Whitney U-test). *p < 0.05; ***p < 0.001.
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Figure 6: Durotaxis of microglial cells. (A) Distribution of microglial cell positions at the end of an experiment. Cells were cultured on compliant substrates (~5 kPa) incorporating a stiffness gradient of ~8 Pa/μm and preferentially migrated toward the stiffer side of the substrate (N = 38). (B) After activation with LPS, microglia durotaxis was significantly enhanced (N = 22). Rose plots were obtained from binned endpoints (bin size: of 60°, with 20° overlap). (C) Schematic plot of microglia migration. Start and end positions are marked with open triangles; black dots indicate cell positions recorded during time-lapse imaging. The blue curve shows the actual migration path (“contour”), the red line indicates the Euclidean distance between start and end position. (D) The directness D was similar for control and LPS-treated cells (p > 0.7, Mann-Whitney U-test). (E,G) Contour velocity vc and Euclidean velocity ve significantly increased after application of LPS (p < 0.05, Mann-Whitney U-test). (F) Stalling phases were highly reduced after LPS treatment (p < 10−4, Mann-Whitney U-test). (H,I) While the component of the Euclidean velocity perpendicular to the mechanical gradient (x-component) was similar for control and LPS-treated cells (p > 0.2, Mann-Whitney U-test), the component toward the stiffer side of the gradient (y-component) was significantly increased after application of LPS (p < 0.05, Mann-Whitney U-test). *p < 0.05; ***p < 0.001.
Mentions: Cells cultured on these compliant substrates migrated comparatively straight; the median directness D (Equation 2), which is the ratio between the shortest linear distance between the start and endpoints of a path and the total travel distance, was 0.73. Furthermore, microglia stopped frequently, with a median stopping rate of 41% (i.e., no movement between 41% of consecutive frames, see Materials and Methods), and traveled with a median contour velocity vc (based on the contour length of the migration path; Equation 3) of 1.48 μm/min, and a median Euclidean velocity ve (based on a straight line between start and end position; Equation 4 and Figure 6C) of 0.82 μm/min (Figure 6). Endpoints of the migration paths showed a non-uniform circular distribution (p < 0.01, Rayleigh test) and were strongly biased toward the stiffer side of the gradient (Figure 6A), indicating that microglial cells showed durotaxis.

Bottom Line: Microglial cells are key players in the primary immune response of the central nervous system.They are highly active and motile cells that chemically and mechanically interact with their environment.Our results demonstrate that microglia are susceptible to mechanical signals, which could be important during central nervous system development and pathologies.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Development and Neuroscience, University of Cambridge Cambridge, UK ; Faculty of Computer Science and Biomedical Engineering, Institute of Biomechanics, Graz University of Technology Graz, Austria.

ABSTRACT
Microglial cells are key players in the primary immune response of the central nervous system. They are highly active and motile cells that chemically and mechanically interact with their environment. While the impact of chemical signaling on microglia function has been studied in much detail, the current understanding of mechanical signaling is very limited. When cultured on compliant substrates, primary microglial cells adapted their spread area, morphology, and actin cytoskeleton to the stiffness of their environment. Traction force microscopy revealed that forces exerted by microglia increase with substrate stiffness until reaching a plateau at a shear modulus of ~5 kPa. When cultured on substrates incorporating stiffness gradients, microglia preferentially migrated toward stiffer regions, a process termed durotaxis. Lipopolysaccharide-induced immune-activation of microglia led to changes in traction forces, increased migration velocities and an amplification of durotaxis. We finally developed a mathematical model connecting traction forces with the durotactic behavior of migrating microglial cells. Our results demonstrate that microglia are susceptible to mechanical signals, which could be important during central nervous system development and pathologies. Stiffness gradients in tissue surrounding neural implants such as electrodes, for example, could mechanically attract microglial cells, thus facilitating foreign body reactions detrimental to electrode functioning.

No MeSH data available.


Related in: MedlinePlus