Block 2. Lecture 8. Cell Communication Flashcards
local signalling
Signals act on nearby target cells
• growth factors such as fibroblast growth factor – FGF1 (paracrine)
• Neurotransmitters such as acetylcholine – Ach (synaptic)
Long-distance signaling
Signals act from a distance
• Hormones produced by specialized cells travel via circulatory system to act on specific cells
eg. insulin from pancreatic beta cells bind to insulin receptors (tyrosine kinase type) initiating a cascade which results in glucose uptake
3 main steps of cell signaling
- Reception( signaling protein(primary messenger) binds to a receptor protein Results in shape and/or chemical state change in the receptor protein.
- Transduction. During the transduction pathway multiple proteins may be activated, typically via phosphorylation.
- Response. Activation of cellular response.
Reception
Signalling protein (primary messenger) binds to a receptor protein Results in shape and/or chemical state change in the receptor protein
Transduction
An altered receptor activates another protein, eg G-protein/adenylyl cyclase
The activated protein (often an enzyme) may cause a relay of changes
Relay molecules are known as “second messengers”, eg. cAMP, IP3
Multiple other proteins may be activated
Each activated protein causes a series of changes, this is often via
phosphorylation – known as a phosphorylation cascade
Response
All of the activated proteins cause one or more functions to occur in the cell
This is where the cell actually does something
how does the signal get to a specific cell
Only certain cells at certain times will have particular receptors, meaning that while the signal might be widespread the transmission of the signal occurs only where it is needed
Intracellular receptors( primary messenger)
Primary messenger is generally hydrophobic and/or small – lipid-soluble can enter the cell
The least common method of signaling
eg. Testosterone, estrogen, progesterone, thyroid hormones bind to receptors within the cytoplasm and move to
nucleus as a complex
Membrane-bound/cell surface receptors:
Primary messenger is generally hydrophilic and/or large
The most common method of signaling
eg. G Protein-Coupled Receptor, Receptor Tyrosine Kinase, ligand-gated ion channel
G-protein coupled receptors( GPCRs)
Transmembrane proteins – pass PM 7 times
Hundreds of different GPCRs exist
Many different ligands Diverse functions:
eg. development, sensory reception (vision, taste, smell)
a target for 1/3 of modern drugs
GPCRs coupled with G proteins
- G proteins are molecular switches that are either on or off depending on whether GDP or GTP is bound
(GTP: guanosine triphosphate, similar to ATP)
The inactive/active state of G proteins depends on
whether GDP(inactive) or GTP(active) bound to the G protein
Steps of G-protein coupled receptors activation
- At rest, the receptor is unbound and G Protein is bound to GDP.
The enzyme is in an inactive state. - Ligand binds to the receptor and binds the G protein. GTP displaces GDP. The receptor and the g-protein are activated. The enzyme is still inactive.
- Activated G protein dissociates from the receptor, and then activates the enzyme to elicit a cellular response.
- G-protein has GTPase activity, promoting its release from the enzyme, and reverting back to its resting state. GTP is hydrolysed back to GDP and P
Steps of G-protein coupled receptors activation
- At rest, the receptor is unbound and G Protein is bound to GDP.
The enzyme is in an inactive state. - Ligand binds the receptor and binds the G protein(to the receptor). GTP displaces GDP(on the g protein). The enzyme is still inactive.
- Activated G protein dissociates from the receptor. The enzyme is activated to elicit a cellular response.
- G protein has GTPase activity(GTP is hydrolyzed back to GDP and P) promoting its release from the enzyme, reverting back to resting state.
Ligand-gated ion channels/receptor
These channel receptors contain a “gate”
Binding of ligand (eg neurotransmitter) at a specific site on receptor elicits a change in shape
channel opens /closes as the receptor changes shape
Ions can pass through the channel (eg. Na+, K+, Ca2+, and/or Cl−)
Ion channel
membrane protein through which specific ions can travel
Ligand
a signalling molecule that binds specifically to another protein
Ion-channel receptor
membrane protein through which specific ions can
travel, in response to ligand binding (also known as ionotropic receptors)
Ligand-gated ion channels/receptors steps
- At rest, the ligand is unbound and the gate is closed.
- Upon ligand binging, the gate opens, specific ions can flow into the cell.
- Following ligand dissociation, the gate closes, back to resting.
Q. Which body system relies heavily on ligand-gated ion channels?
The nervous system:
– released neurotransmitters bind as ligands to ion channels on target cells to propagate action potentials
Signal Transduction Pathways.(phosphorylation cascade)
Signals relayed from receptors to target molecules via a ‘cascade’ of molecular interactions
Series pf protein kinases each adding a phosphate to the next kinase
*typically it is serine or threonine(aminoacids) that are phosphorylated. This means that mutations affecting these residues could be detrimental,
Protein kinase
Protein kinases are enzymes that transfer a phosphate group from ATP to another (specific) protein. Typically, this activates the protein.
Phosphatases
Phosphatases are enzymes that dephosphorylate (remove the phosphate) rendering the protein inactive, but recyclable
Use of a second messenger
Sometimes another small molecule is included in the cascade, these are second messengers.
eg. cAMP and calcium ions
From GPCR :
The activated enzyme is adenylyl cyclase
Activated adenylyl cyclase converts ATP to cAMP
cAMP acts as a second messenger and activates downstream protein (which could be the start of a phosphorylation cascade)
*this pathway is disrupred by cholera toxin
Calcium
Widely used second messenger
Low [Ca2+] inside cell (typically ~100nm)
Very high [Ca2+] outside the cell (more than 1000-fold higher)
Maintenance of concentration via calcium
pumps are important as high [Ca2+] can damage cells!
increased concentration of Ca2+ inside the cytosol leads to cell response
Common Ca2+ pumps/channel sites
- out of cell
- into ER
- into mitochondria
Ca2+ and IP3 in GPCR signalling
Here, the activated protein is phospholipase C which then cleaves PIP2 (a phospholipid) into DAG and IP3
IP3 diffuses through the cytosol and binds to a gated channel in the ER
Calcium ions flow out of ER down a concentration gradient and activate other proteins towards a cellular response( Ca2+ here is technically a 3rd messenger but is called second)
*Muscle use Ca2+ to contract
Why do the cellular response activation happens in so many steps?
Amplifies the response
Provides multiple control points
Allows for specificity of response(temporal, spatial)
despite molecules in common
Allows for coordination with other signaling pathways
Examples of cellular response include:
- Gene expression
• Alteration of protein function to gain or lose an activity
• Opening or closing of an ion channel
• Alteration of cellular metabolism
• Regulation of cellular organelles or organisation
• Rearrangement/movement of cytoskeleton
• A combination of any of these
The transduction of a signal leads to the regulation of one or more cellular activities
How is the response turned off?
All of the signals are for a limited time: activation usually promotes the start of deactivation, so that signalling is of short period of time, ensuring homeostatic equilibrium.
it means the cell is ready to respond again if required
cAMP is broken down by phosphodiesterase (PDE)
Example: caffeine blocks the action of PDE
Inhibition of specific PDE’s can also be a therapeutic approach
eg Viagra - inhibits a specific cGMP-degrading PDE
Adrenaline stimuation of glycogen breakdown
Adrenalin acts through a GPCR, activates cAMP and two protein kinases in a phosphorylation cascade
Results in active glycogen phosphorylase which can convert glycogen to glucose 1-phosphate
Amplification means that 1 adrenalin molecule can result in 10^8 glucose 1-phosphate molecules!( amplification occurs at every step)
glycogen breakdown
Glycogen is a long term energy store in liver and skeletal muscle
glycogen breakdown results in glucose 1-phosphate
glucose 1-phosphate is then converted to glucose 6-phosphate which can then be used in glycolysis to generate ATP
Target cell
A Cell is a target cell if it has a receptor for that ligand
The two common amino acids that tend to be impacted by phosphorylation
Typically serine and threonine residues that are being phosphorylated
If you have a DNA level mutation, the residues don’t end up being serine or threonine, the ability of that protein to be phosphorylated has been affected
Second messenger
small non-protein( eg calcium)
tyrosine kinase receptor
plasma membrane receptor
A kinase is an enzyme that catalyzes the transfer of phosphate groups.
- The binding of a signaling molecule (eg growth factor) causes two receptor monomers to associate closely with each other, forming a complex known as a dimer.
- dimerization activates the tyrosine kinase region of each monomer, each tyrosine kinase adds a phosphate from an ATP molecule to a tyrosine on the tail of the other monomer.
- The fully activated receptor is recognized by specific relay proteins inside the cell. Each such protein binds to a specific phosphorylated tyrosine, undergoing a resulting structural change that activates the bound protein, Each activated protein triggers a transduction pathway, leading to a cellular response.
Activation of a protein/protein kinase in the phosphorylation cascade is often associated with
a change in molecular shape
cAMP stands for
cyclic adenosine monophosphate
Cholera mechanism of action
cholera bacterium forms a biofilm on the lining of the small intestine and produces a toxin. The cholera toxin is an enzyme that chemically modifies a G protein involved in regulating salt and water secretion. The modified g protein is unable to hydrolyze GTP to GDP, it remains in the active form, constantly stimulating adenylyl cyclase to make cAMP. Causes the intestinal cells to secrete; large amounts of salts into the intestine, the water follows by osmosis. Loss of fluids
