Background

Directed cell migration in response to various types of gradients plays a crucial role in physiological and pathological conditions. Depending on the cue that is responsible for directed migration, several different types of “taxis” have been reported in the literature such as chemotaxis (i.e. movement in response to a chemical stimulus), haptotaxis (i.e. movement in response to adhesive substrates), rheotaxis (i.e. movement in response to currents), curvotaxis (i.e. movement in response to cell scale curvature variations), topotaxis (i.e. movement in response to signaling pathways activated by the extracellular matrix, and the control of cell stiffness), and mechanotaxis (i.e. movement in response to mechanical cues). A mechanical cue known to influence cell migration is the gradient of substrate elastic modulus. However, the elastic modulus alone cannot fully define the material properties of the cellular microenvironment, as thisoften has both elastic and viscous characteristics. While the reported literature discusses the effect of uniform viscoelasticity on cell migration, the in vivo microenvironment is rarely uniform. In their work, Shirke et al investigated the influence of viscous nature (loss modulus, G’’) on cell migration, and defined a newly reported form of cellular migration known as Viscotaxis.

Key findings and developments

            The authors began by finding acrylamide and bis-acrylamide concentrations that could give rise to polyacrylamide gels with similar storage modulus (a measure of the elastic response of the material), but different loss moduli (measure of the viscous response of the material). Following multiple characterizations, they selected two compositions that give rise to gels with the same storage modulus but widely different loss moduli- which they termed High Loss (for high loss modulus), and Low Loss (for low loss modulus). They then prepared a gel with a gradient of loss modulus using two drop technique, based on diffusion and polymerization, which allowed rheological characterization by quantifying and visualizing the gradient strength.

            Following the generation of the suitable substrates, the authors went on to explore whether the gradient of loss modulus induces directed cell migration. For this, they recorded the movement of human mesenchymal stem cells on substrates with loss modulus gradient, using time-lapse microscopy. Using this method, they reported movement of the majority of cells from High Loss to Low Loss- namely, viscotaxis. This type of migration was not observed on gels with uniform loss modulus, rather than one having a gradient. Further quantification of the tactic index confirmed that over 70% of cells respond to the modulus gradient, while 30% remain unresponsive even across time. To estimate the strength of the migration bias, the authors calculated the average cell displacement over time, and found that biased migration took about 6h to get established. Moreover, they found that more randomness in cell migration exists in Low Loss as compared to that on High Loss. Attempting to further characterize the reason for unresponsiveness on 30% of cells, the authors explored whether the initial location of a cell on a gradient determines its decision, and found this is not the case, altogether suggesting that there is a sub-population of cells that are non-responsive to a given gradient strength.

            The authors then investigated the influence of the strength of loss modulus gradient on migration bias. They generated a gel with a step increase in loss modulus and determined 4 possible cell trajectories, grouped into two types of responses- the first, positive response (following the viscotaxis gradient) involved a) cells starting in the High Loss modulus region, and crossing the boundary into the Low Loss modulus region and b) cells starting in the Low Loss modulus region and moving towards the High Loss modulus region, but instead taking a U-turn. A positive response was observed in 83% of cells. The second set of trajectories involve a negative response (contradicting the predicted viscotaxis gradient), namely a) cells crossing from the Low Loss to the High Loss modulus region, and b) cells starting in the High Loss modulus region, approaching the boundary of the Low Loss modulus region, and taking a U-turn at the boundary. Atomic force microscopy showed there was no gradient in the elastic modulus.

            Finally, the authors went on to investigate the mechanisms of viscotaxis. For this, they measured cellular traction on uniform Low Loss and High Loss substrates using traction force microscopy. They found that cells apply higher traction force on more elastic materials (i.e. materials with low loss modulus), which might result in force asymmetry, and biased displacement of cells causing viscotaxis. Upon chemical disruption of actomyosin contractility, directional migration was lost. Moreover, using HeLa cells, the authors showed that cells can maintain a stable morphology on High and Low Loss substrates, but that due to the inability to build up traction, in High Loss substrates the cell boundaries fluctuate more.

 

What I like about this preprint

             I like the interdisciplinarity of the work. Moreover I think we still know very little about all the biophysical aspects mediating cell migration. The authors conclude their preprint stating that this is one more aspect to consider when considering tissue bioengineering. I think the more we understand these biophysical aspects, the closer we will be to reproducing important aspects in vitro in increased efforts to replace in vivo models.

Open questions

1. Have you explored the role of viscotaxis in an integrated approach as would be observed in vivo– namely, including chemotactic, durotactic, etc. gradients? What would be the main differences you would expect, in an integrated approach?

2. What would be the role of viscotaxis under flow conditions – for instance if this were to play a role in circulating cells within the vasculature?

3. You used two different cells types for different purposes in this study. Would you expect different behaviours with respect to viscotaxis across multiple different cells which would be representatives of various cells in the body? Would you expect something very different to what you observed in mesenchymal stem cells?

4. One important aspect you discuss in your paper is the relevance of this finding for preparing stiffness-based substrates for the field of tissue engineering. As organs-on-chip and organoids gain momentum, how do you envisage integrating knowledge on the biophysical properties of substrates with in vivo and in vitro studies?

References

1. Shirke PU et al, “Viscotaxis”-directed migration of mesenchymal stem cells in response to loss modulus gradient, bioRxiv, 2020.

 

doi: https://doi.org/10.1242/prelights.26007

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