revealing-the-cytoskeletal-organization-of-invasive-cancer-cells-in-3d

Revealing the Cytoskeletal Organization of Invasive Cancer Cells in 3D

Cell migration has traditionally been studied in 2D substrates. However, it has become increasingly evident that there is a need to study cell migration ...

Institut Curie

Interactions between dynamic microtubules and actin filaments (F-actin) underlie a range of cellular processes including cell polarity and motility. In growth cones, dynamic microtubules are continually extending into selected filopodia, aligning alongside the proximal ends of the F-actin bundles. This interaction is essential for neuritogenesis and growth-cone pathfinding. However, the molecular components mediating the interaction between microtubules and filopodial F-actin have yet to be determined. Here we show that drebrin, an F-actin-associated protein, binds directly to the microtubule-binding protein EB3. In growth cones, this interaction occurs specifically when drebrin is located on F-actin in the proximal region of filopodia and when EB3 is located at the tips of microtubules invading filopodia. When this interaction is disrupted, the formation of growth cones and the extension of neurites are impaired. We conclude that drebrin targets EB3 to coordinate F-actin-microtubule interactions that underlie neuritogenesis.

Targeting of the F-actin-binding protein drebrin by the microtubule plus-tip protein EB3 is required for neuritogenesis.

Interactions between dynamic microtubules and actin filaments are essential to a wide range of cell biological processes including cell division, motility and morphogenesis. In neuronal growth cones, interactions between microtubules and actin filaments in filopodia are necessary for growth cones to make a turn. Growth-cone turning is a fundamental behaviour during axon guidance, as correct navigation of the growth cone through the embryo is required for it to locate an appropriate synaptic partner. Microtubule-actin filament interactions also occur in the transition zone and central domain of the growth cone, where actin arcs exert compressive forces to corral microtubules into the core of the growth cone and thereby facilitate microtubule bundling, a requirement for axon formation. We now have a fairly comprehensive understanding of the dynamic behaviour of the cytoskeleton in growth cones, and the stage is set for discovering the molecular machinery that enables microtubule-actin filament coupling in growth cones, as well as the intracellular signalling pathways that regulate these interactions. Furthermore, recent experiments suggest that microtubule-actin filament interactions might also be important for the formation of dendritic spines from filopodia in mature neurons. Therefore, the mechanisms coupling microtubules to actin filaments in growth-cone turning and dendritic-spine maturation might be conserved.

Cytoskeletal dynamics in growth-cone steering.

Formation of a functional nervous system requires neurons to migrate to the correct place within the developing brain. Tangentially migrating neurons are guided by a leading process which extends towards the target and is followed by the cell body. How environmental cues are coupled to specific cytoskeletal changes to produce and guide leading process growth is unknown. One such cytoskeletal modulator is drebrin, an actin-binding protein known to induce protrusions in many cell types and be important for regulating neuronal morphology. Using the migration of oculomotor neurons as a model, we have shown that drebrin is necessary for the generation and guidance of the leading process. In the absence of drebrin, leading processes are not formed and cells fail to migrate although axon growth and pathfinding appear grossly unaffected. Conversely, when levels of drebrin are elevated the leading processes turn away from their target and as a result the motor neuron cell bodies move along abnormal paths within the brain. The aberrant trajectories were highly reproducible suggesting that drebrin is required to interpret specific guidance cues. The axons and growth cones of these neurons display morphological changes, particularly increased branching and filopodial number but despite this they extend along normal developmental pathways. Collectively these results show that drebrin is initially necessary for the formation of a leading process and subsequently for this to respond to navigational signals and grow in the correct direction. Furthermore, we have shown that the actions of drebrin can be segregated within individual motor neurons to direct their migration independently of axon guidance.

Drebrin controls neuronal migration through the formation and alignment of the leading process.

During metastasis, cancer cells breach the basement membrane and migrate through the stroma mostly composed of a network of collagen I fibers. Cell migration on 2D is initiated by protrusion of the cell membrane followed by formation of adhesions that link the actin cytoskeleton to the extracellular matrix (ECM). Cells then move forwards by exerting traction forces on the adhesions at its front and by disassembling adhesions at the rear. In 2D, only the ventral surface of a migrating cell is in contact with the ECM, where cell-matrix adhesions are assembled. In 3D matrices, even though the whole surface of a migrating cell is available for interacting with the ECM, it is unclear whether discrete adhesion structures actually exist. Using high-resolution confocal microscopy we imaged the endogenous adhesome proteins in three different cancer cell types embedded in non-pepsinized collagen type I, polymerized at a slow rate, to allow the formation of a network that resembles the organization of EMC observed in vivo. Vinculin aggregates were detected in the cellular protrusions, frequently colocalizing with collagen fibers, implying they correspond to adhesion structures in 3D. As the distance from the substrate bottom increases, adhesion aggregates become smaller and almost undetectable in some cell lines. Using intravital imaging we show here, for the first time, the existence of adhesome proteins aggregates in vivo. These aggregates share similarities with the ones found in 3D collagen matrices. It still remains to be determined if adhesions assembled in 3D and in vivo share functional similarities to the well-described adhesions in 2D. This will provide a major step forward in understanding cell migration in more physiological environments.

Do cancer cells have distinct adhesions in 3D collagen matrices and in vivo?

Drebrin is an actin filament (F-actin)-binding protein with crucial roles in neuritogenesis and synaptic plasticity. Drebrin couples dynamic microtubules to F-actin in growth cone filopodia via binding to the microtubule-binding +TIP protein EB3 and organizes F-actin in dendritic spines. Precisely how drebrin interacts with F-actin and how this is regulated is unknown. We used cellular and in vitro assays with a library of drebrin deletion constructs to map F-actin binding sites. We discovered two domains in the N-terminal half of drebrin-a coiled-coil domain and a helical domain-that independently bound to F-actin and cooperatively bundled F-actin. However, this activity was repressed by an intramolecular interaction relieved by Cdk5 phosphorylation of serine 142 located in the coiled-coil domain. Phospho-mimetic and phospho-dead mutants of serine 142 interfered with neuritogenesis and coupling of microtubules to F-actin in growth cone filopodia. These findings show that drebrin contains a cryptic F-actin-bundling activity regulated by phosphorylation and provide a mechanistic model for microtubule-F-actin coupling.

Drebrin contains a cryptic F-actin-bundling activity regulated by Cdk5 phosphorylation.

Deciphering the multifactorial determinants of tumor progression requires standardized high-throughput preparation of 3D in vitro cellular assays. We present a simple microfluidic method based on the encapsulation and growth of cells inside permeable, elastic, hollow microspheres. We show that this approach enables mass production of size-controlled multicellular spheroids. Due to their geometry and elasticity, these microcapsules can uniquely serve as quantitative mechanical sensors to measure the pressure exerted by the expanding spheroid. By monitoring the growth of individual encapsulated spheroids after confluence, we dissect the dynamics of pressure buildup toward a steady-state value, consistent with the concept of homeostatic pressure. In turn, these confining conditions are observed to increase the cellular density and affect the cellular organization of the spheroid. Postconfluent spheroids exhibit a necrotic core cemented by a blend of extracellular material and surrounded by a rim of proliferating hypermotile cells. By performing invasion assays in a collagen matrix, we report that peripheral cells readily escape preconfined spheroids and cell-cell cohesivity is maintained for freely growing spheroids, suggesting that mechanical cues from the surrounding microenvironment may trigger cell invasion from a growing tumor. Overall, our technology offers a unique avenue to produce in vitro cell-based assays useful for developing new anticancer therapies and to investigate the interplay between mechanics and growth in tumor evolution.

Article	Year
Targeting of the F-actin-binding protein drebrin by the microtubule plus-tip protein EB3 is required for neuritogenesis. Nature cell biology\| PubMed ID: 18806788	2008
Cytoskeletal dynamics in growth-cone steering. Journal of cell science\| PubMed ID: 19812305	2009
Drebrin controls neuronal migration through the formation and alignment of the leading process. Molecular and cellular neurosciences\| PubMed ID: 22306864	2012
Do cancer cells have distinct adhesions in 3D collagen matrices and in vivo? European journal of cell biology\| PubMed ID: 22939225	2012
Drebrin contains a cryptic F-actin-bundling activity regulated by Cdk5 phosphorylation. The Journal of cell biology\| PubMed ID: 23979715	2013
Cellular capsules as a tool for multicellular spheroid production and for investigating the mechanics of tumor progression in vitro. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 23980147	2013

Sara Geraldo

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