The cytoskeleton and cell-cell junctions

Importance of multicellularity

• cells cooperate and specialise

• Allows exploitation of resources which single celled organisms can’t

Importance of cell interactions

• cells assemble tissues

• Cells communicate

Cytoskeleton

• internal rigging, provides mechanical strength

• Drives organelle movement

• Anchor for cell-cell junctions

• Determines cell polarity

• Role in mitosis

• Cell movement

Structure of microtubules

• stiff tubes

• Alpha and beta subunits

• Non covalent heterodimers

• + and - ends

Microtubules subunits

• alpha and beta

• Bind GTP and GDP

GTP and microtubules

• GTP bound heterodimers bind at + end

• Converted to GDP upon growth

Rapid shrinkage of microtubules

• GDP bound heterodimers bind to + end

Dynamic instability of microtubules

• allows rapid restructuring

• Adaptation

Microtubules function

• spindles in metaphase

• Cytokinesis- restructures internal networks

• Positions and moves organelles

Actin filament structures

• Polymer actin monomers

• Monomers bind ATP, converted to ADP in filaments

• 2 filaments twist to form an actin molecule

• + and - end

Function of actin filaments

• grows from both ends

• More flexible than microtubules

• Bundle together , causes different cell elements (strong)

• Controls cell shape and movement

Intermediate filament structure

• rope-like fibres

• Alpha helix monomers

• InterMediate filament proteins

• Coils

• Filaments

• Non nucleotide binding proteins (differs from other 2 components)

• Provide mechanical strength

Impacts of dynamic cytoskeleton

• subunit assembly and disassembly leads to growth or shrinkage of filaments

• Allows for rapid structural reorganisation

• Elements form stable cell-cell junctions

What does the dynamic cytoskeleton influence

• cell-cell comms

• Cell organisation

• Developmental choices

Cell-cell adhesions

• Cell-cell anchoring junctions

• Cell matrix anchoring junctions

• Tight junctions

Examples of cell-cell anchoring junctions

• adherents junction

• Desmosomes

Desmosomes

• contain specialised cadherins

• Connect intermediate filaments

• Allows cells to stay together under high mechanical stress

Adherens Junction

• allows indirect linkage of actin cytoskeletons to neighbouring cells

• Cadherins form homodimers in the protein

• Extracellular has cadherin repeats separated by Ca ion binding sites

• The binding of N terminal cadherin domains is calcium dependent

Cadherins

• very large family

• Specialised transmembrane proteins

• Form homodimers

• Cadherins of the same type on adjacent CM interact weakly

• Abundance of weak interactions = Velcro effect

Effect of calcium loss on cadherins

• cells become isolated (cannot bind to other n terminal cadherin domains)

Evidence that only homotypic recognition occurs

• sponge cells disaggregated through fine sieve

• Cultured in sea water

• Hybrids of 2 cells wont wont, cells of same type aggregate

• No heterotypic binding

Effect of expression of specific cadherins

• selective recognition

• Cadherins recognise identical cadherins

Why is species selective recognition important

• vital in developmental tissue assembly

Species selective recognition proof

• if a tissue is disaggregated and mixed up

• The cells will recognise where they belong and re aggregate accordingly

Adherens junction function

• coordinate actin based motility of cells

• Cadherin interactions indirectly link actin cytoskeletons of adjacent cells via adaptor / anchor proteins

• Form a belt around epithelial cells

Adherens junctions forming belt around epi cells

• continuous contractile belt around epi cells

• Multiple connections = transcellular network, allows tissue to start behaving as tissue

• Can tighten, oxbow lake style formation of epithelial tubes

Cell- matrix anchoring junctions function

• bind cells to molecules of the ECM

Focal adhesions (cell-matrix anchoring junctions)

• fibronectin

• Connect ECM to cytoskeleton

Integrin structure (cell-matrix anchoring junctions)

• alpha and beta subunits form a heterodimer

• Links ECM to intracellular protein (talin)

• Talin links to actin

How do integrins bind lots of ligands

• many different alpha and beta chains

• Many different combos

• Each has a different ligand binding property

How do integrins compare to cadherins

• bind ECM molecules w low affinity in abundance (Velcro)

• Interactions can be formed and released

How does integrin allow cell migration

• switches from active to inactive form

Importance of cell-matrix anchoring junctions

• cells need to anchor to ECM to receive signals

• Lack of anchorage = cell stress or death (can’t receive signals)

ECM anchorage dependence

• mediated by integrins and cascade signals generated

What is an important role of integrins

• Important developmental role

What are tight junctions comparable to

• stitching

What do tight junctions do

• seal gaps between epi cells

Occluding tight junctions

• maintains cell polarity

• Blocks mixing of basolateral and apical membranes

• The membranes have separate unique protein pools for their differing functions

What membranes are separated by occluding tight junctions

• basolateral and apical

Cell polarity

• functional importance

• Allows unidirectional transport of glucose from the gut lumen to the blood

• AT of glucose from apical membrane

• Passive transport from basolateral

• Transporters are separated

How is cell polarity maintained (occluding tight junctions)

• Bands of CM proteins encircling cell

• tight extracellular seal by rows of proteins from neighbouring cells interlocking

• Proteins rows prevent lateral diffusion of proteins and lipids in CM

Channel forming junction example

• Gap junction

Gap junction

• connexons from adjacent plasma membranes align (homotypic or heterotypic channels)

• Allows metabolic or electrical coupling

• Smooths out fluctuations in conc of small molecules in neighbouring cells interlocking Proteins

• Coordinates cell response

• Allows AP to spread rapidly

Connexon structures

• 6 connexins