New insights into directed cell movement

Our body heals an accidental injury, a knife cut for example, within a few minutes. Have you ever wondered how that happens? Immune cells migrate to the site of injury and subsequently heal the injury. The active migration of blood cells towards the cut site is an example of directed migration. But how do cells sense and respond to directional cues like an injury? Two studies conducted at the Marine Biological Laboratory (MBL), USA comprising researchers from National Centre for Biological Sciences, India, National Institutes of Health, USA and Harvard Medical School, USA have provided new insights into the process.

Cell migration is a key process in the development and maintenance of multicellular organisms. Wound healing and immune responses are some of the processes that require the directed movement of cells. Proteins present on the cell's surface, called integrins, sense external cues like injury and transmit the information to the cell's internal machinery. The actin filaments form long protein chains and along with motor proteins, generate forces that ultimately result in the propulsion of the cell towards the external cue.

The integrins reside within aggregations of multiple proteins called focal adhesions. Contraction at the rear end, or the part of the cell away from the cue, and the release of focal adhesion complexes help in the migration of cells towards the external signal. Thus, cell migration involves extensive communication between the cell's surface and its interior. An important link in this communication are the integrins. The integrins transduce the force generated inside the cell to the substrate outside the cell and facilitate cell movement.

In an activated state

Focal adhesions have been shown to respond to directional physical cues. However, how is this information tranduced to the cell at a molecular level? Do integrins provide directional information and could they function as the elusive 'direction encoder' that operate within focal adhesions? Previous studies of integrins have identified an 'activated state' ­- a structural conformation where integrin proteins are fully extended and can easily bind to the substrate. However, how this active state is induced is still not understood.

Though several competing theoretical models of integrin activation exist, the current study provides credible evidence of the 'cytoskeletal force model'. This model states that a force exerted by the actin cytoskeleton is required for extending and activating the integrin molecule. Since actin flow is directional, the coupling of this flow to the activation status of the integrin molecule could encode directional information in the cell.

For the study, the researchers used highly motile fibroblast cells from mouse as a model system. To study the organisation of integrin proteins on fibroblasts' surface during migration, the team fused a constraint fluorescent protein to integrin. This enabled visualising the orientation of integrin molecule within focal adhesion complexes. The authors found that integrin molecules within a focal adhesion protein complex are co-aligned.

Previous studies have shown that when the cell extends its cell membrane to move forward, actin filaments inside the cell move in the opposite direction (retrograde flow). This retrograde flow generates the necessary forces that facilitate cell movement. To test if actin retrograde flow is required for proper integrin alignment, the researchers blocked the flow using inhibitors. They found that in regions where the retrograde actin flow was completely blocked, the integrins alignment was altered. In regions inside the cell, where actin flow remained intact, integrins were aligned as in the normal migrating cells.

The polarisation microscopy images gave a 2D projection of integrin alignment on the cell membrane. The researchers next used Rosetta modelling to infer the 3D orientation of integrin proteins inside focal adhesions. They found that not only was the integrin aligned in the direction of actin flow, but it also adopted a tilted conformation with respect to the substrate surface. This showed a kind of a conformational change that is required for the protein to achieve the activated state.

The researchers also looked at the orientation of integrins in fast migrating immune cells and observed a similar correlation between the alignment of integrins and the direction of actin flow at the leading edge of the migrating cells. Taken together, these studies show that directional actin retrograde flow inside the cell is conveyed to the integrin proteins on the cell membrane, which result in the activation and co-alignment of integrins within the focal adhesions and facilitate directed cell movement.

(The author is with Gubbi Labs, a Bengaluru-based research collective)