ECM, Integrins and Cell Polarity Flashcards
Describe cells and anchor points
Cells need anchor points to generate force. Some structures associated with myosin motors, some not.
Parallel bundles, meshwork of actin, connections to adhesion sites
Describe the stages of cell migrations
Describe the ECM
Not found in unicellular organisms. All ECMs have functions in cell adhesion, cell signalling and cell differentiation.
Tissue specific, e.g. plant ECM includes cell wall components and signalling molecules.
Functions of the animal ECM
- Support for cells
- ECM regulates: polarity, cell division, adhesion, motility
- Development: Migration highway, differentiation cues, growth factors
What are the two types of matrix?
Basement membrane (basal lamina) and interstitial matrix
What are the two main classes of macromolecules within the ECM?
Fibrous proteins (glycoproteins) and polysaccharide chains (which link to core proteins to form proteoglycans)
Matrix in bone
Rock-hard and dense, calcified
Matrix in the cornea
Made of a soft, transparent extracellular matrix
Where is the interstitial matrix found?
Connective tissues
What secretes the ECM in most connective tissues?
Fibroblasts.
ECM molecules are synthesised intracellularly and then secreted by exocytosis. Composition varies enormously depending on the tissue. Form huge, organised aggregates outside the cell
Describe collagen
Provides tensile strength in animal connective tissue
- Mammals have ~20 collagen genes
- Collagens 1 and 2 (fibrillar) are important for interstitial
- Collagen makes up ~25% of protein mass in a mammal
- Cells secrete collagen in the form of pro collagen to prevent intracellular aggregation
- Procollagen is extensively processed by various enzymes
What does incorrect collagen assembly give rise to?
Hyper-extensible skin
- Genetic defects give rise to a disease called Ehlers-Danlos syndrome
- Collagen fibrils do not assemble correctly
- Elastic skin
- Also problems with wound healing and scarring
How is compression resisted?
Polysaccharide and protein gels fill the spaces and resist compression
- Function is complementary to collagen
- Proteoglycans are extracellular proteins linked to glycosaminoglycans
- Can have many GAG chains attached to a single core proteins linked
- Proteoglycans and GAGs can form enormous macromolecules - “bottlebrushes” - with molecular weight of millions of daltons
- Forms gels that, when combined with collagen, leads to a matrix that is tough, resilient and resistant to compression
Basement membrane
Underlies all epithelia. Thin, sheet like structures that underlie all epithelial cell layers in the animal body
More in-depth of how basement membrane underlies all epithelia
- Thin sheets of tissue that anchor the basal side of epithelial cells to deeper tissues
- Separates these cells from surrounding connective tissue
- Forms the mechanical connection between them
- Important for cell polarity, cell survival, cell proliferation and differentiation
- Highway for cell migration
- Can act as a selective filter
Name the several molecules that make up a basement membrane
Laminin (primary organiser)
- Nidogen
- Fibronectin
- Perelman
- Hyaluronan
- Decorin
- Aggrecan
What do Perelman and nidogen do?
Link laminin and type 4 collagen together
Describe laminin
- Primary organisation of the sheet
- Multi-domain glycoprotein composed of 3 polypeptides
- Disulfide-bonded into an asymmetric cross
- Several types of alpha, beta and gamma chains, forming up to 45 laminin isoforms
- Tissue specificity
- Self-assembly into a network via head domain interactions
- Interaction with cells is needed to organise this into a sheet (adhesion receptors)
Describe collagen IV
- Type IV collagen is an essential component of mature basement membranes
- 3 separately synthesised protein chains that come together to form a triple helix
- Interations between the N-terminal domains creates a flexible network
Describe how the col IV and laminin network is linked
- Laminin generates the initial sheet
- Type IV collagen on top of the
- Nidogen and Perlecan link the two polymers
- Adhesion receptors protruding from the plasma membrane on the base lateral side of the epithelium network join the whole basement membranes together
How is the ECM connected to the cell?
Via receptor proteins
- Cells make ECM, organise it and secrete enzymes to degrade it.
- matrix in its turn affects the cells using transmembrane cell adhesion proteins
- Different types of matrix receptors
Describe during migration
Focal adhesions form at the cell front and disassemble at the rear.
- Cell adhesions connect the cell to the substrate at the cell front
- Adhesions are what enables the force to be generated to move the cell body during cell migration
- Cell adhesions also provide signalling cues to a variety of different cell types that depend upon the type of extracellular matrix for various cellular functions
- Integrins are important for transmitting these signals
Describe integrins
Receptors that couple the matrix to the cytoskeleton - cellular go-between
- Transmembrane heterodimers
- 24 mammalian integrins
- Usually link to actin via the ABPs talin and vinculin
- Binding of integrins to the matrix allows tension generation between the matrix and the cell
- The integrins are connected to the actin cytoskeleton
- When the cytoskeleton contracts, tension created pulls the cell forward
- Correct connections and binding strength are essential
Describe a nascent adhesion
A few proteins link integrins to the actin cytoskeleton.
- Newly forming focal contact.
- Composed if only a few proteins at the start
- Integrins on the plasma membrane recognise an ECM component
- Integrin ECM engagement
- Integrin clustering
- Association with other proteins to form a focal contact
Describe how the composition of the matrix organises FA distribution
Slide 38
Key points of integrins
Newly forming focal contact with integrin, vinculin, talin and actin
Association with myosin II for contractile activity/generation of some force
Allows formation of mature focal adhesion.
This is a hub for cell signalling and membrane trafficking
Describe bi-directional signalling of integrins
- Bind to a wide variety of molecules
- Interactions with ICAMs (outside cell) or talin (inside cell) activates the integrin
- Both cue outside conformational change, with higher affinity for ECM
- Triggers internal signal cascades that change cell polarity, brace cytoskeletal structure and encourage cell survival and proliferation
- Focal adhesion can transmit chemical as well as mechanical signals across the cell membrane - accessory molecules
- Complex signalling pathways that regulate cell survival, differentiation, migration
How do multiple individual molecular components collaborate to produce complex cell behaviours
- Cells polarise in response to extracellular cues of intracellular cues or intracellular landmarks to establish specific domains
- Coordination with cytoskeleton is required for a cell to build different structures with distinct molecular components at the front vs back or top vs bottom
- Oriented cell divisions in tissues
- Genetic studies in yeast, flies and worms have provided most of our current understanding of the molecular basis of cell polarity
- Many molecular components have been evolutionarily conserved
What is polarity?
Functional or morphological asymmetry relative to one or more axes
- Most cells are polarised
- Ultimately derives from the polarity of macromolecular complexes in the cells
Forms of polarity
- Polarity in prokaryotes
- Polarity in fixed-shape single-celled eukaryotes
- Dynamic polarity during eukaryotic cell migration - Epithelial polarity
- Neuronal polarity
- Oocyte polarity
- Planar cell polarity
- Describe polarity in prokaryotes
Many cells are elongated and show clear polarity.
- Important to orient cell divisions, help infection in some infectious forms, sporulation, swimming
- Membrane curvature can regulate localisation in some bacteria
Describe localisation of ActA
Asymmetric.
- ActA protein accumulates at one pole only
- ActA accumulates at younger poles during cell divisions
- Polarity in chlamydomonas
- Centrioles can determine the polarity of the whole cell
- Two cilia in fixed orientation
- All organelles organises along axes of polarity
- Dynamic polarity during eukaryotic cell migration
Tested using defined fibronectin micro patterns.
Extracellular matrix geometry determines the orientation of polarity axes.
In animals, polarity depends on the extracellular environment.
- Actin, golgi, nucleus always in a similar position
- Nucleus-centrosome-Golgi axis directed towards cell adhesive edges
- Dependent on shape of micropattern
- Apical basal polarity of epithelial cells in the intestine
Bundled actin filaments form microvilli that increase surface area on the apical side
Microtubules run from top to bottom of the cell and provide a global co-ordination system
Describe PAR proteins
Organise the polarity of cells in relation to their neighbours and basal lamina
- Scribble and Crumbs
- Influence each others activity (by phosphorylation) and localisation at cell-cell junctions
- Bind and position small GTPases
- Localisation of mRNA can establish cell polarity in embryogenesis
- mRNA localisation is widespread
- 20% of all mRNA are localised in fly ovaries
- Allows loci, regulated translation
- Prevents translation in other parts of the cell
Establishment of the body axes by mRNA localisation
- bicoid mRNA localises to the anterior pole of the oocyte
- Oscar mRNA localises to the posterior pole
- they are required to establish the adult body axes
- mRNA localisation requires mRNA transport, anchoring, translational repression and regulated localised translation
- Planar cell polarity (PCP)
Orientation of specific structures within the plane of an epithelium
What is a central regulator of cell polarity?
Cdc42
