L15: Cell migration Flashcards
what is cell migration?
Many cells move by ‘crawling’ either through tissues or along surfaces.
* e.g., gastrulation during embryogenesis involves cells moving over the surface
of the blastocyst.
* Neural crest cells move long distances through the developing embryo from
the neural tube to a variety of sites.
* Macrophages and neutrophils move from the blood stream in to surrounding
tissues in response to chemokines released from sites of injury/infection.
* Growth cones at the ends of neuronal axons migrate large distances through
the organism.
* Fibroblasts migrate through (and often remodel) connective tissue in order to
repair damaged/injured tissue
* Metastatic cancer cells migrate from sites of primary tumour formation to
form secondary metastases elsewhere in the organism.
modes of migration?
- Mesenchymal Migration
Cells migrate through collagen matrices using integrin-mediated adhesion.
Cells attach to ECM via integrins, degrade ECM using proteases (e.g., MMPs), and move forward.
Example: Fibroblasts moving during wound healing. - Collective Migration
Cells move as a group, maintaining cell-cell junctions while interacting with the ECM.
The leading edge cells use integrins to interact with the ECM.
Follower cells remain connected through cadherins or tight junctions.
Example:
Gastrulation (movement of embryonic cells).
Neural crest cell migration (cells migrating from the neural tube to form various tissues). - Amoeboid Migration
Cells squeeze through collagen gaps without binding to ECM (no need for integrins).
Movement is driven by cytoskeletal contraction and shape changes rather than ECM degradation.
Example:
Neutrophils (immune cells) move toward bacteria using chemotaxis.
Neutrophils detect a gradient of formyl-methionine-leucine-phenylalanine (fMLP), a bacterial signal.
They migrate toward the source, catch the bacteria, and engulf them via phagocytosis.
mesenchymal migration?
Fibroblast migration. Chemotaxis of fibroblasts to chemotactic source (image). Fibroblast polarises towards chemoattractant source forming a lamellipodium- thin sheet like foot? Movement of lamellipodium towards chemotactic. Rear of cell attached by focal adhesion is released, creates a propulsive force (actin-myosin contraction at rear) forces cell body forward creates more protrusion in that direction.
2D VS 3D migration?
2d vs 3d migration
Most cases- 3d process
2d-cells form triangular structures on average for a fibroblasts, have alemmlipodia with f actin criss crossed in a structured pattern and cell moves along the surface. In 3d in a 3d collagen matrix for instance, no flat structure, more of a blob with a pseudobod sticking out with fingers sending collagen etc. for directionality? On collagen-ccoated glass no soluble gradient present unless have microneedle. Matrix is stiff. Glass itself is stiff. No ability to pull collagen, stuck to glass. Focal adhesion restricted to c y plane where this cell is attached to collagen. Cell forms fried egg like structure whereas in 3d there are soluble gradients of broken down collagen, proteins, other molecules, no polarity, discrete matrix firbrils that can be pulled on, spreading in migration are hindered by collagen matrix whereas in 2d matrix underneath the cell so no constraints to move. Focal adhesions are all over the surface not constrained to x y axis, gel itself has low stiffness.
actin?
- Spatial and temporal regulation of actin polymerisation and
actomyosin-based contraction are fundamental to cell migration. - Different actin structures are present, and these depend on specific
regulatory signals that act spatially and temporally to enable cell
movement.
migration cycle?
Migration cycle
* Actin-polymerisation dependent
protrusion pushes the
membrane forward
* Contraction at rear of cell
creates a hydrodynamic force
that pushes the cell forward
* New adhesions made at the
front of the cell, whilst old ones
at the rear are disassembled.
Barbed actin filaments are localised near the plasma membrane, driving protrusion. membrane thought to be vibrating, creates space for new actin monomer to add onto actin filament, preventing the membrane from retracting backward. Actin polymerization at the leading edge pushes the plasma membrane forward, causing protrusions like lamellipodia or filopodia to extend.
Barbed actin filaments add new actin monomers, which push the membrane forward and drive protrusion.
At rear of cell creates hydrodynamic pressure pushes cell forward as well as driving movement of g actin monomers that result from actin depolymerisation at the end of the cell. Not passive by diffusion, hydrodynamic force. To front of cell for polylermisation.
New focal adhesions made at new protrusion to stabilise connection of that protrusion to substrate. As the cell moves, Old focal adhesions disasemble to enhance propulsive force at back of the cell. Ensures rear is free to retract and continue generating forward movement. These processes happen simultaneously.
Front and back coordinated.
Stress fibres through cell connected at the back and front of the cell provide their own force via actin myosin contraction.Friction between the focal adhesions and the substrate counteracts the traction force, balancing it to maintain controlled movement.
regulation of actin turnover?
formin- nucleates assembly and remains associated with the growing plus end.
thymosin- binds subunits, prevents assembly.
tropomodulin- prevents assembly and dissasembly at minus end (stabilisation)
cofilin- binds adp-actin filaments, accelerates dissassembly
arp2/3- nucleates assembly to form a web and remains associated with the minus end
profilin- binds subunits speeds elongation
tropomyosin- stabilises filament
capping proteins- prevents assembly and disasembly at plus end
gelosin- severs filaments and binds to plus end
reg actin turnover description
Nucleation and Formins:
ARP2/3 complex and formin proteins are essential for actin filament nucleation. The ARP2/3 complex helps initiate branches from pre-existing actin filaments, while formins promote linear filament elongation by adding actin monomers at the barbed (+) end.
Polymerization at the Barbed End:
Formins and profilin are critical for polymerization from the barbed (+) end. Profilin binds to G-actin (globular actin) in the ATP-bound state, facilitating its addition to the growing filament at the barbed end. This is correct.
Instability of Actin Filaments:
Actin filaments are unstable and require various proteins to stabilize them. For example, capping proteins bind to the barbed (+) end, preventing further polymerization or depolymerization at that site, while tropomyosin stabilizes the filament by binding along its sides.
Tropomodulin and Tropomyosin:
Tropomodulin binds to the pointed (-) end, preventing disassembly by inhibiting depolymerization.
Tropomyosin stabilizes actin filaments by binding along the sides, preventing severing by proteins like gelsolin and inhibiting depolymerization by cofilin. For gelsolin to sever the filament or for cofilin to depolymerize it, tropomyosin must first be removed.
Profilin and Thymosin:
Thymosin-β4 sequesters G-actin in its ADP-bound state, preventing polymerization.
Profilin exchanges ADP for ATP on G-actin, converting it to the ATP-bound form, which promotes polymerization at the barbed end. This exchange mechanism is crucial for maintaining the pool of ATP-actin available for filament growth.
Gelsolin Severing Actin Filaments:
Gelsolin severs actin filaments by breaking them into smaller fragments. When gelsolin severs a filament, it creates two new barbed (+) ends (one at each of the new fragments), which speeds up actin turnover by increasing the number of polymerization and depolymerization sites. This increases the rate of G-actin recycling.
G-Actin Pool and Polymerization:
The severing of actin filaments by gelsolin increases the number of barbed (+) ends available for polymerization and the number of pointed (-) ends available for depolymerization, which accelerates the overall turnover of actin filaments. By constantly recycling G-actin, cells can rapidly remodel their actin cytoskeleton.
Key Clarifications:
Gelsolin doesn’t just split a filament into two smaller filaments; it also creates more polymerization sites by exposing new barbed (+) ends, increasing the rate of actin filament turnover.
Tropomyosin needs to be removed for gelsolin to sever actin filaments because it inhibits the action of gelsolin and other severing proteins.
Profilin plays a critical role in actin polymerization by enabling the exchange of ADP for ATP on G-actin, preparing it for incorporation into the growing filament.
arp2/3
Actin-Related Protein 2/3 (ARP2/3)
ARP2/3 complex consists of 7 subunits, including ARP2 and ARP3, which are required for actin nucleation.
The complex is usually inactive until an activating factor binds, causing a conformational change in ARP2 and ARP3, enabling binding to G-actin monomers.
G-actin binds to the ARP2/3 complex via its pointed end, and the complex caps this end, preventing disassembly.
New filaments grow from the barbed end, and the ARP2/3 complex is positioned at the pointed end of the new filament.
Filament growth occurs at an angle of 70 degrees relative to the original filament, creating a dendritic network.
arp2/3 driving protrusion
Actin polymerization occurs at the leading edge of the cell, where ARP2/3 nucleates new filaments.
Cofilin promotes disassembly of actin filaments further from the leading edge.
Thymosin β4 sequesters G-actin and helps push it toward the front of the cell, where it is bound by Profilin.
Profilin facilitates the addition of G-actin to the growing barbed end of new filaments.
The polymerization of actin filaments at the leading edge creates a propulsive force that pushes the plasma membrane (PM) forward.
As the filaments grow, the ARP2/3 complex dissociates and is recycled to the front of the cell, where it begins nucleating new filaments again, sustaining the directional movement.
