Frictiotaxis underlies adhesion-independent durotaxis

Background:

Cell motility is a complex process, and as such, there are several factors that one needs to consider when thinking about it. First of all, whether cells have the innate ability to move or not: most cells do, but there is a huge variability. Then, what kind of motility the cells are capable of: do they move like an amoeba, or do they use specific anchorage to a surface. Going even further, what is the environment of cells: is it a 2D or a 3D environment, and can cells move without restrictions or do they need to squeeze through confined spaces. On top of all this, what sort of cues/signals do cells use to find their way: are these soluble chemical signals (chemotaxis), surface bound chemical signals (haptotaxis), or is it a difference in stiffness (durotaxis).

Broadly speaking, we can use the above points to describe cell motility, a feature analogous to crawling, walking, or running. Here, we are not even talking about cells using specialized structures, like a flagellum, which they can use for swimming in a medium, or cells using cues such as a source of light (phototaxis) to guide themselves. So, where does this preprint fit into all this?

The study described in this preprint focuses on motility along surfaces. Cells are known to use durotaxis in such environments, which requires strong and specific attachment to the surface, which in turn is required to “sense” the difference in surface stiffness across the length of the cell. As the authors mention, this would preclude durotaxis in cells that use amoeboid movement, i.e., cells which are not capable of producing specific surface attachments. In this preprint the authors decided to challenge this view and asked, can cells sense surface stiffness without specific surface attachments, and if so, how?

Key findings:

One of the first points the authors make in this preprint is that it was challenging to “fabricate confined cellular environments of tunable stiffness that are necessary to study adhesion-independent migration” in 3D. So, the authors devised a microchannel setup, made of agarose gel, which can be tuned to have “substrates of physiologically relevant stiffnesses”, where the channel is wide enough for the cell to crawl through without difficulty, while being narrow enough to produce the effect of confinement. This setup also produced a gradient of stiffness along the length of the microchannel to test durotaxis.

Passivating the microchannel with polyethylene glycol (PEG) prevented cells from producing specific surface adhesion, and the authors used this treatment to assess durotaxis while the experimental cell lines performed amoeboid motility. The authors could show that cells move up the stiffness gradient, and confirmed that this occurred without a specific adhesion, seeing the absence of Paxillin staining which labels focal adhesion structures. The authors noted that cells entering the microchannel with a stiffness gradient showed greater persistence in their movement up the stiffness gradient; while cells entering a microchannel without stiffness gradient showed a tendency to move back-and-forth, as if they couldn’t “decide” which way to go.

Having shown that cells can durotax without specific adhesion to the substrate, the authors wondered how this was possible. One possibility they considered was that the effect was only correlative and not causal. They hypothesized that cells were, in fact, sensing the friction along the surface, given that the surface friction and substrate stiffness would be correlated. They formally tested this correlation using ‘lateral force microscopy’ (LFM), allowing frictional forces to be measured. This is analogous to dragging a stick over a surface to guess how slippery vs. sticky the surface is.

At this point, the authors introduced a mathematical model to describe how cell motility can be achieved using a friction gradient. The amoeboid movement of a cell relies on movement of the actomyosin meshwork: the actomyosin meshwork flows to the rear end of the cell, “squeezes” the cell there, propeling the other end of the cell forward, and the cycle repeats in a loop as long as the cell keeps moving. At the beginning of such movement, when the front or rear end is not specified, different parts of the cell will compete to be the next rear end, and whichever part wins will propel the cell towards the front. The tie breaker, in this case, is the flow of actomyosin. In the model described by the authors, friction will dictate which side of the cell will have slower flow, making that side the front, and thus indirectly specifying which side will be the rear end. Cells thus move up a stiffness/friction gradient, due to the influence of friction outside the cell, on the actomyosin flow inside the cell.

The prediction of the model was clear: if a cell is given a friction gradient, it will sense it and move up the gradient. The authors coated the surface of the microchannel with opposing surface gradients of PEG (passive non-sticky substance) and bovine serum albumin (BSA) (non-specific sticky substance) to produce microchannels with friction gradients, and duly characterized this gradient using the LFM. When cells entered the microchannel, they moved up the friction gradient: demonstrating “frictiotaxis”, for the first time.

Importance of the findings:

Identifying the environmental cues that guide cell motility has been a long-standing research interest. This preprint demonstrates, for the first time, that cells are capable of sensing the difference in surface friction, at a length-scale comparable to cell dimensions, to guide their motility. Given that the substrate stiffness and surface friction are correlated, various tissues present different stiffness in vivo, and the experiments use a physiologically relevant stiffness range, it is likely that cells use frictiotaxis under physiological conditions as well.

The authors highlight some open questions in the field: it remains unclear how cells prioritize and integrate various environmental cues to regulate their motility and find their target. This is like going on a treasure hunt, with a lot of clues, but without knowing which are the useful ones. Stretching the analogy even further, the clues might be present in a “language” that we might not even understand, and it is interesting to figure out if there are differences in the capacity of cells to interpret the clues in different “languages”, or if cells simply ignore some clues, and are biased towards some other ones. The authors argue that immune cells are a promising candidate for using frictiotaxis, as they “often need to traverse tissues of high density to reach target sites. Adhesion-independent durotaxis could contribute significantly to this type of directed migration”. Having tested the adhesion-independent durotaxis in the ‘HL60 neutrophil-like cells’, this manuscript takes the first steps towards testing the importance of durotaxis in immune cells’ motility in vivo.

Questions to authors:

1) In your experiments, the cells move up a friction/stiffness gradient in microchannels, i.e., in a 3D environment. It is likely that the cells are able to produce strong enough forces perpendicular to the surface to generate frictional forces, but only if they are in a 3D environment. In this context, what is the role of confined environment in frictiotaxis? Would the model of frictiotaxis predict that a cell would similarly movement up a friction/stiffness gradient if it were on a 2D surface?

2) It is clear that ‘frictiotaxis’ is ‘adhesion-independent durotaxis’. But, what do you think would be the difference between ‘frictiotaxis’ and ‘adhesion-independent haptotaxis’, as both require a gradient of surface bound molecules?

3) From what I understand, tumors typically have greater stiffness than the neighbouring tissue. In such a scenario, would you predict that durotaxis or frictiotaxis is trying to prevent tumor metastasis? Would you imagine that cancerous cells would need to overcome this barrier, and the other migration cues need to be strong enough for the cells to migrate out of the tumor?

Tags: cell motility, durotaxis, frictiotaxis, micro-fabrication, neutrophil-like cells

doi: https://doi.org/10.1242/prelights.34966

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