Intercellular Signal Trafficking Flashcards
Describe paracrine signalling
Cells exchange signalling factors with each other in order to organise themselves into a functioning unit such as a tissue
What are transport mechanisms crucial for determining?
- Signalling range
- Signalling speed
- Select target cells
What are the four transport mechanisms for paracrine signals?
- Diffusion
- Transcytosis
- Exovesicles
- Cytonemes
Describe the definition of diffusion
Definition of diffusion:
- movement of a component through
space under the influence of a physical
stimulus.
- The most common cause of diffusion is
a concentration gradient.
- The flux goes from region of high
concentrations to regions of low
concentration
- Therefore, diffusion adjusts the
component concentration until it
reaches equilibrium.
- Described in Fick’s first law
- In short, diffusion is the physical flow
of material.
Describe the challenges of diffusion in a tissue
- Secretion from the source cell
- Interaction with the matrix
- Interaction with receptors and uptake
in the target cells - Degradation of ligand
- (Release of the ligand from target cell
What are some characteristics of a diffusion-based transport in a tissue?
- Long life-time of the ligand in the extracellular space
- Short intracellular life-time of the ligand (-> degradation within minutes)
- Limited control about concentration of ligand at target cell
- No control about direction
- Long-range signalling requires a long duration
How does the extracellular matrix influence diffusion?
1 - The hindered diffusion
* The ligand diffuses in a complex
environment
* The signal finds obstacles (cells)
2 - The ligand - matrix interaction is complex
e.g. binding to
* Polysaccharides (Heparin)
* Proteoglycans
* Glycolipids
* EC Proteins (receptors, adhesion,…)
3 - The ligand can also interact with cells:
* The ligand binds/unbinds to receptors
* Ligand can be modified or taken up by
cells
Describe the extracellular matrix and proteoglycans
- Proteoglycans are proteins modified with
many polysaccharide (sugar) side chains. - PS chains are covalently attached to the
protein, which is often anchored to the
membrane) - 95% of the proteoglycan consists of
polysaccharides - PGs are produced in the ER
- PGs are anchored to the surface of the cells
and travers through the cell membrane - PGs are a main component of the cartilage
- PGs are involved in signalling (co-receptors)
- PGs regulate in embryonic development and
wound healing
Describe proteoglycans and the regulation of the Fgf signalling pathway
Heparin sulphate proteoglycans (HSPGs) are
components of the ECM.
Fgf (fibroblast growth factors) are a family of
cell signalling proteins that are mainly involved
in development and regeneration.
Fgfs are
released locally upon injury.
HSPGs have been shown to:
* mediate Fgf signal transduction by acting as
co-receptors
* regulate Fgf receptor trafficking to and from
the plasma membrane
* control the secretion of Fgf ligands
* signal to other cells by acting as cues or
presenting Fgf ligands to their receptors
* regulate the structure of the ECM and the
establishment of the Fgf signaling gradients
i.e. by influencing hindered diffusion.
Describe the restrictive clearance model (RCM)
The RCM describes the transport of a ligand THROUGH a tissue, which reduces
extracellular ligand concentration. This implies continuous degradation to
establish/maintain a concentration gradient.
Hypothesis of the RCM: the rate of uptake/degradation regulates the gradient.
- Decreased uptake/degradation rates lead to long-range spreading.
- Increased uptake/degradation rates lead to short-range spreading.
Describe the Fgf8 example of RCM
Fgf8 is important for regulating cell
proliferation and fate determination.
Rab5 is a GTPase and its activity
correlates with endocytic uptake
B) Blockage of Rab5 activity (by RN-tre)
leads to enhanced signalling range.
C) Increased Rab5 activity reduces
signalling range.
Describe planar transcytosis
Transcytosis describes the transport of a ligand THROUGH the cells. This implies a
continuous uptake, degradation and recycling with subsequent secretion.
Hypothesis of the planar transcytosis model: the rate of endocytosis/recycling
regulates the spreading.
- Decreased endocytosis/recycling rates lead to short-range signalling.
- Increased endocytosis/recycling rates lead to long-range signalling.
Describe the Dpp example of Planar transcytosis
Decapentaplegic: signalling factor, which
belongs to the TGFβ growth factor family.
Dpp is important for regulating patterning
and cell fate determination in the
Drosophila wing.
Rab5 activity correlates with endocytic
uptake:
d-f - Reduced Rab5 (by DN-Rab5S43N) activity
reduces Dpp signalling range.
g-i - Activation of Rab5 activity
(overexpression) leads to enhanced Dpp
signalling range.
The consequence of enhanced uptake of the signalling molecules can be seen in increased
induction of target genes. This tells us that the signalling gradient has been increased.
Describe exovesicles in signalling biology
- Exosomes are a type of
exovesicles - Diameters ranges from 30 –
100nm (isolation by
ultracentrifugation) - Lipid bilayer
- Contain a specific subset of
➢ lipids (i.e. Cholesterol,
sphingomyelin)
Cargo
➢ nucleic acids (i.e. miRNA)
➢ cellular proteins (i.e. from
endosomes, PM, cytosol)
➢ signalling proteins (Wnt, Shh)
Describe the exosome secretion pathway
[Image on slides]
* ESCRT is a intracellular sorting machinery of four complexes
* ESCRT-0: recognizes and sequesters TM proteins in the endosomal membrane
* ESCRT-1,2 is responsible for membrane deformation into buds with sorted cargo
* ESCRT-3 / Vps4: drives vesicle scission
Describe cytonemes
Cables for intercellular communication
Thin, finger-like projections
- similar to filopodia
- generally thinner and more
elongated.
- can reach lengths of up to 150µm
Specialization for cell signaling:
- transport of signaling molecules
between cells.
- tips of cytonemes contain
receptors
- contain an internal core of actin
filaments
Directed growth
- can grow rapidly in a directed
manner towards target cells.
Dynamic structures
- can constantly extend, retract,
and branch
Describe filopodia
- Finger-like, cytoplasmic projections
that extend from the cell membrane. - contain a few dozens actin filaments
cross-linked into bundles by actinbinding proteins - Filopodia are Cdc42 dependent
- Typical length ca 5-35µm
Function :
* movement of cells
* sensing and probing the environment
* important in cell-cell interactions:
* cytoneme
Describe the biology of filopodia
1 - Initiation phase
* Filopodia are initiated by bending of the
membrane by the Bar-proteins IRSp53, Toca1.
* Subsequently, factors of the nucleation complex
such as the Formins, N-WASP, and RhoGTPase
Cdc42 become active.
2 - Protrusion phase
* Cdc42 activity leads to formation of actin
filaments (F-actin), which stabilize the
protrusions.
* Fascin interconnect actin filaments to dense
bundles.
3 - Disassembly phase
* Filopodia shrink by Cofilin-mediated actin
depolymerisation and filament disassembly
initiated in tip regions and along their shafts.
Describe the convergence-elongation model of filopodia extension
- Microspikes extend from lamellipodia and are
transformed in filopodia. - These filopodia are anchored into the F-actin
dendritic meshwork. A push of the F-actin bundles
leads to the elongation of filopodia. - The filopodium grows from the base
- Problem: Removal of Rac1 lamellipodia does NOT
affect CDC42 filopodia format
Describe the filopodia tip/nucleation model of filopodia extension
- Filopodia tip complex (IRSp53, MyoX, Toca1,
Cdc42) forms at the plasma membrane - The tip complex facilitates de novo filopodia
formation and elongation. Formins regulate
actin polymerisation. - The filopodium grows from the tip.
- Problem: G-actin has to be rapidly
transported to the tip
Describe conclusions from the two models of filopodia extension
Conclusion: There are several ways to form filopodia and these are probably cell
type specific. Signalling filopodia – cytonemes - form mainly through nucleation
Describe cytonemes in signalling filopodia in flies
Three examples for a cytoneme based transport
in invertebrates. Cytonemes are well studied in
the wing imaginal disc of Drosophila:
- Dpp and its receptor thick veins (Tkv) signal
over the dorso-ventral border. - FGF signals regulate the formation of tracheae.
Cells from the air sac primordium (ASP) extend
cytonemes to take up FGF signal from the FGF
source cells. - Hh and its co-receptor Smoothened (Smo) can be
found on cytonemes. The span across the AP
boundary. Smo is expressed in the anterior
compartment, whereas Hh in the posterior part.
Describe cytonemes in vertebrates
- Initiation phase
* Wnt protein clusters at the cell membrane
* Co-localisation with IRSp53 and Cdc42. - Protrusion phase
* Wnt protein is localised at the tip of
cytonemes.
* Cdc42 activity leads to formation of
actin filaments, which stabilize
cytonemes. - Signalling phase
* After connection to the target cell,
Wnt binds to the receptor
* The Wnt cytoneme is cut
of/fragments.
* Wnt signals remains at the target cell
membrane.
* Wnt signalling is initiated.
Describe cytonemes in determining the anteroposterior patterning in the neural plate
- Wnt cytonemes generate a Wnt/βcatenin gradient in the neural plate
- Long cytonemes ‘posteriorise’ the
neural plate -> no forebrain - Short cytonemes lead ‘anteriorise’ the
neural plate -> increased forebrain and
eyes
Describe shh cytonemes in limb development
- The Shh gradient in the limb bud
operates during hand primordium
development . - From low concentrations to high
concentrations forms thumb (1) to little
finger (5) - Shh is transported on long, thin
cytonemes from producing cells (purple
with green Shh protein) to target cells
(red cells). - These cytonemes are up to 200µm
long! The opposing filopodia directly
interact : Shh and its receptor Smo can
bind (white brackets).
Describe cytonemes forming between different cell types
- Direct communication: Neurons interact
with cancer cells via cytonemes - Cancer regulation: These interactions
influence cancer growth, spread,
treatment resistance, and immune
response. - Therapeutic potential: Cytoneme synapses
may offer new cancer treatment
strategies.
