Block 2. Lecture 8. Cell Communication

Question 1

local signalling

Answer 1

Signals act on nearby target cells
• growth factors such as fibroblast growth factor – FGF1 (paracrine)
• Neurotransmitters such as acetylcholine – Ach (synaptic)

Question 2

Long-distance signaling

Answer 2

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

Question 3

3 main steps of cell signaling

Answer 3

1. Reception( signaling protein(primary messenger) binds to a receptor protein Results in shape and/or chemical state change in the receptor protein.
2. Transduction. During the transduction pathway multiple proteins may be activated, typically via phosphorylation.
3. Response. Activation of cellular response.

Question 4

Reception

Answer 4

Signalling protein (primary messenger) binds to a receptor protein Results in shape and/or chemical state change in the receptor protein

Question 5

Transduction

Answer 5

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

Question 6

Response

Answer 6

All of the activated proteins cause one or more functions to occur in the cell
This is where the cell actually does something

Question 7

how does the signal get to a specific cell

Answer 7

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

Question 8

Intracellular receptors( primary messenger)

Answer 8

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

Question 9

Membrane-bound/cell surface receptors:

Answer 9

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

Question 10

G-protein coupled receptors( GPCRs)

Answer 10

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)

Question 11

The inactive/active state of G proteins depends on

Answer 11

whether GDP(inactive) or GTP(active) bound to the G protein

Question 12

Steps of G-protein coupled receptors activation

Answer 12

1. At rest, the receptor is unbound and G Protein is bound to GDP.
   The enzyme is in an inactive state.
2. 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.
3. Activated G protein dissociates from the receptor, and then activates the enzyme to elicit a cellular response.
4. 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

Question 12

Steps of G-protein coupled receptors activation

Answer 12

1. At rest, the receptor is unbound and G Protein is bound to GDP.
   The enzyme is in an inactive state.
2. Ligand binds the receptor and binds the G protein(to the receptor). GTP displaces GDP(on the g protein). The enzyme is still inactive.
3. Activated G protein dissociates from the receptor. The enzyme is activated to elicit a cellular response.
4. G protein has GTPase activity(GTP is hydrolyzed back to GDP and P) promoting its release from the enzyme, reverting back to resting state.

Question 13

Ligand-gated ion channels/receptor

Answer 13

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−)

Question 14

Ion channel

Answer 14

membrane protein through which specific ions can travel

Question 15

Ligand

Answer 15

a signalling molecule that binds specifically to another protein

Question 16

Ion-channel receptor

Answer 16

membrane protein through which specific ions can

travel, in response to ligand binding (also known as ionotropic receptors)

Question 17

Ligand-gated ion channels/receptors steps

Answer 17

1. At rest, the ligand is unbound and the gate is closed.
2. Upon ligand binging, the gate opens, specific ions can flow into the cell.
3. Following ligand dissociation, the gate closes, back to resting.

Question 18

Q. Which body system relies heavily on ligand-gated ion channels?

Answer 18

The nervous system:

– released neurotransmitters bind as ligands to ion channels on target cells to propagate action potentials

Question 19

Signal Transduction Pathways.(phosphorylation cascade)

Answer 19

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,

Question 20

Protein kinase

Answer 20

Protein kinases are enzymes that transfer a phosphate group from ATP to another (specific) protein. Typically, this activates the protein.

Question 21

Phosphatases

Answer 21

Phosphatases are enzymes that dephosphorylate (remove the phosphate) rendering the protein inactive, but recyclable

Question 22

Use of a second messenger

Answer 22

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

Question 23

Calcium

Answer 23

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

Question 24

Ca2+ and IP3 in GPCR signalling

Answer 24

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

Question 25

Why do the cellular response activation happens in so many steps?

Answer 25

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

Question 26

Examples of cellular response include:

Answer 26

• 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

Question 27

How is the response turned off?

Answer 27

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

Question 28

Adrenaline stimuation of glycogen breakdown

Answer 28

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)

Question 29

glycogen breakdown

Answer 29

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

Question 30

Target cell

Answer 30

A Cell is a target cell if it has a receptor for that ligand

Question 31

The two common amino acids that tend to be impacted by phosphorylation

Answer 31

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

Question 32

Second messenger

Answer 32

small non-protein( eg calcium)

Question 33

tyrosine kinase receptor

Answer 33

plasma membrane receptor
A kinase is an enzyme that catalyzes the transfer of phosphate groups.

1. 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.
2. 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.
3. 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.

Question 34

Activation of a protein/protein kinase in the phosphorylation cascade is often associated with

Answer 34

a change in molecular shape

Question 35

cAMP stands for

Answer 35

cyclic adenosine monophosphate

Question 36

Cholera mechanism of action

Answer 36

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