Polarized cell migration in the fast lane

Directed cell migration has special importance in processes such as cancer metastasis, immune surveillance and development. In these different processes, cell migration enables cancer cells to reach blood vessels for metastasis, immune cells to move to a site of infection, or the production of morphogenetic events that pattern our body. Understanding this process is therefore crucial to target these different processes in different path-physiological conditions. During cell motility, cells dynamically connect their cytoskeleton to adhesion molecules that interact with the extracellular matrix (ECM). This is a highly choreographed process that enables the mechanical forces necessary for movement to be produced. The ability to move in a specific direction in response to extracellular cues requires the establishment of distinct mechanical modules,that produce the forces to push at the front, and to pull at the back of the cell, to produce net movement. A major question is how signaling activities position these different functional modules in space and time to produce polarized cell migration.

HFSP Program Grant holders Olivier Pertz, Gaudenz Danuser, Noo Li Jeon and colleagues
authored on Fri, 07 November 2014

In novel work published in Developmental Cell, a collaborative effort between the groups of Olivier Pertz (University of Basel, Switzerland), Gaudenz Danuser (Harvard Medical School, USA) and Noo Li Jeon (Seoul National University, Republic of Korea), funded by an HFSP Program Grant, provides novel insight into how robust polarized cell migration is spatially regulated. Cell migration has been classically studied on glass coverslips, which conveniently allows imaging using light microscopes. However, an important caveat is that because of too much adhesion, and lack of any directional cue, large mesenchymal cells such as fibroblasts cannot migrate directionally, precluding the study of polarized cell motility. To solve this issue, fibroblasts were plated on 20 µm wide ECM highways that mimic the ECM fibrils present in vivo. On this highway, fibroblasts only exhibited transient polarized episodes of cell migration, leading them to “wiggle” left and right. However, when stimulated with a specific growth factor called PDGF, fibroblasts then “chose” a specific direction, and robustly migrated in this direction for long periods of time. Thus, mimicking the geometry of ECM present in vivo enables robust polarized cell migration to be induced.

Figure: Polarized, persistent cell migration system. A. Phase contrast time series of a polarized fibroblast. Note unidirectional cell migration. B. Cytoskeletal organization in polarized fibroblasts. A front module consisting of podosome structures (green dots at the front) enables membrane protrusion, while a back myosin module (red cluster) allows for tail retraction.

Two important properties of the system can explain the emergent property of robust polarized cell migration. First, at the cell front, the fibroblasts produce dot-like cytoskeletal structures, called podosomes.  These have a precise lifetime of exactly 10 minutes, and therefore enable the cell to build a podosome zone of which the size and position with respect to the cell front is dynamically maintained throughout the cell migration process. The cell therefore uses these cytoskeletal structures to build a “dynamic internal ruler” that spatially positions signaling activities necessary for cell polarization. Second, we found that the podosome zone locally inhibits a signaling molecule called RhoA, which is a key regulator of cellular contractility. This led to the specific inhibition of contractile actin cables called stress fibers at one pole of the cell. In cells without podosomes, front/back mechanical linkage through these cables only allows episodic polarization events. In polarized cells, the podosome zone breaks this linkage, and “mechanically insulates” the front and the back of the cell, which is essential for long term polarized cell migration.

The HFSP-funded grant enabled the multidisciplinary approach necessary for this work. The Pertz lab performed the live imaging of multiple cytoskeletal markers as well as biosensors to study spatio-temporal control of signaling dynamics. The Danuser lab designed novel computer vision algorithms to track cell morphodynamics at multiple time scales, which provided the basis for understanding and quantifying the mechanical insulation of front and back. The Jeon lab engineered the microfabrication technology necessary to build the ECM highways. Efforts from the three teams were required for the successful completion of the project. Our results provide direct biophysical insight into how signaling is spatially regulated to position different mechanical modules to initiate and maintain polarized cell migration. This might provide novel concepts to target directed cell migration during pathophysiological conditions such as metastatic cancer or inflammation.

Reference

A growth factor-induced, spatially organizing cytoskeletal module enables rapid and persistent fibroblast migration. Martin K, Vilela M, Jeon NL, Danuser G, Pertz O. Dev Cell. 2014 Sep 29;30(6):701-16. doi: 10.1016/j.devcel.2014.07.022.

Pubmed link