Lecture 4: Cell to Cell interactions

What are the three stages common to signal transduction pathways and what happens in each stage?

Reception: a ligand binds and activates a receptor. Transduction: receptor activation initiates signalling cascades where secondary messengers pass the signal from molecule to molecule. Response: transcription factors are activated and alter gene expression.

What is meant by paracrine signalling?

signalling molecules act on nearby cells rather than distant targets.

What structural feature do the TGF-β superfamilies ligands share?

dimeric, two subunits may be identical or different.

What are the two major branches of canonical TGF-β signalling?

TGF-B/Activin/Nodal branch signalling through Smad2/3. BMP branch signalling through Smad1/5/8. Both signal through Smad4.

What role does Smad4 play in TGF-β signalling?

acts as common co-Smad, forms complexes with activated Smads, allows signalling from both branches to regulate transcription

What are some functions of TGF-β family members in development?

cell division, embryonic patterning, mesoderm formation, left-right asymmetry and tissue development.

What developmental roles were specifically associated with Activins and Nodals?

mesoderm differentiation and left-right asymmetry

Describe the receptor complex formed during TGF-β signalling reception.

One dimeric ligand, two Type I receptors and two Type II receptors.

What type of enzymes are TGF-β receptors and how are they activated?

They are serine/threonine kinases. Type II receptors phosphorylate Type I receptors in the GS domain, activating them.

What is phosphorylation in this context?

addition of phosphate groups to proteins, altering their activity.

What happens during the transduction phase of Smad signalling?

Activated Type I receptors phosphorylate Smad proteins, which act as secondary messengers.

What occurs during the response stage of Smad signalling?

Activated Smads form complexes, move into the nucleus, tissue-specific transcription factors regulate target gene expression.

What common principle of signalling pathways was highlighted in the lecture?

Signalling often involves changes in subcellular localisation, where signalling components move between cellular compartments to become activated.

What is non-canonical TGF-β signalling?

pathways other than Smads, may cause diff response

How is BMP signalling mainly controlled?

BMP signalling is controlled primarily through inhibition.

What are ligand traps and how do they regulate signalling?

bind signalling molecules before they reach receptors, prevent pathway activation, ensure signalling occurs only at correct time and place

Give examples of BMP ligand traps and explain their developmental importance.

Chordin, Noggin and Follistatin. Regulate signalling, contribute to dorsal fate specification

How can pathway activity be experimentally visualised?

detecting phosphorylated Smads such as pSmad1/5.

How can TGF-β signalling be experimentally manipulated?

Mutated receptors generated, kinase-dead cause LOF, constitutively active cause GOF

What is meant by loss-of-function and gain-of-function mutations?

Loss-of-function reduces or removes activity, while gain-of-function causes increased or continuous activity.

Why are model organisms useful for studying signalling pathways?

allow signalling pathways to be manipulated experimentally, help explain clinically relevant biological mechanisms

Why is Smad2/3 signalling important during embryonic development?

essential for mesoderm formation.

Which molecules mediate Smad2/3 signalling in vivo?

Nodal ligands

What does the oep mutant experiment show us?

oep (mutated Nodal co-receptor) mimicked loss of squint/cyclops, injecting with type I receptor mRNA fixed, showed defect high up in pathway

What are receptor tyrosine kinases (RTKs)?

large family of cell surface receptors involved in signalling, 58 genes into 20 subfamilies.

What are the structural features of RTKs?

RTKs possess extracellular ligand-binding domains and intracellular tyrosine kinase domains.

In what state do RTKs usually exist before activation?

monomers, except for insulin receptor

Describe canonical RTK activation.

Ligands usually dimerise, promoting receptor dimerisation or oligomerisation then cross-phosphorylation of kinase domains.

What are the effects of RTK cross-phosphorylation?

It increases kinase activity, stabilises receptors in the active state and creates docking sites for signalling proteins.

How do phosphorylated RTKs initiate downstream signalling?

create docking sites recognised by proteins containing SH2 domains.

How does Ras function as a signalling switch?

switches between inactive GDP-bound state and active GTP-bound state.

What roles do Sos and GRB2 play in Ras signalling?

GRB2 links receptors to Sos, Sos acts as GEF

Describe the Ras/MAPK signalling cascade downstream of RTKs.

Active Ras stimulates Raf, stims. MEK, stims. MAPK, downstream cellular affects

How many FGF ligands and receptors are present?

There are 22 FGF ligands but only four receptors: FGFR1, FGFR2, FGFR3 and FGFR4.

What structural features of FGFRs are important for ligand binding?

Contain Ig-like domains D1-D3, ligands primarily bind to D2 and D3.

What are HSPGs and what is their structure?

Heparan sulfate proteoglycans, protein core attached to long heparan sugar chains that can undergo sulphation modifications.

Why are HSPGs important in FGF signalling?

facilitate FGF-FGFR interactions, sulphation patterns create specificity, tethers paracrine FGF to cell surface helping to localise

Why do paracrine and endocrine FGFs behave differently?

Paracrine FGFs have high affinity for HSPGs and remain local, whereas endocrine FGFs have lower affinity and diffuse into the bloodstream.

How can FGF signalling produce different cellular behaviours?

Different downstream pathways activated, resulting in proliferation, survival or motility.

Why are signalling pathways clinically important?

can cause achondroplasia (short limbed dwarfism), Pfeiffer syndrome and Apert syndrome (skull fusion and limb abnormalities).