Lecture 8

Question 1

Why do cells communicate

Answer 1

Cells need to be able to respond as a cell, as a part of a whole tissue, and as an organism for function - basically just think back to the levels of organisation
They respond to signals from other cells and from the environment
These signals are often chemical (but can also be sound or light - from environment)

Question 2

Describe Local signalling

Answer 2

Signals act on nearby target cells
Examples:
Growth factors such as fibroblast growth factor - FGF1 (paracrine)
Neurotransmitters such as acetylcholine – ACh (synaptic)

Question 3

Long distance signalling

Answer 3

Signals act from a distance
- Hormones produced by specialised cells travel via circulatory system to act on specific target cells
E.g. insulin from pancreatic beta cells to insulin receptors (tyrosine kinases) initiating a cascade which results in glucose uptake.
(this is called endocrine signalling)

Question 4

What are target cells

Answer 4

Target cells are cells that have receptors (reminder they are proteins) which can respond to certain (e.g. chemical) messengers

Question 5

Describe and name the 3 main steps of cell signalling:

Answer 5

Reception
Transduction
Response

Cell signalling is where a ligand is used to activate (not enter through) the protein which also activates further proteins and in doing so eventually elicits a cell response

Question 6

Describe receptor

Answer 6

a molecule/protein which responds to a specific ligand

Question 7

Describe ligand

Answer 7

a signalling molecule that binds specifically to another protein

Question 8

Describe reception

Answer 8

Ligand or Signalling molecule (or First/Primary messenger) binds via the binding site to a receptor protein.
Results in shape and/or chemical state change in the receptor protein

Question 9

Describe transduction:

Answer 9

Altered receptor (e.g. GPCR) activates another protein (always at least a few) - e.g. G protein/adenylyl cyclase)
The activated protein (often an enzyme) may cause a relay of changes
Relay molecules known as “second messengers” - eg. cAMP, IP3 - may be produced
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 10

Describe response:

Answer 10

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

Question 11

Describe receptor specificity

Answer 11

Receptors are very specific: only the target receptor on the target cell will interact with that signal (ligand) and use it to activate signal transduction pathways (the lock and key analogy).
We have specific receptors because of the differing 3D molecular shape of the proteins involved - structure determines function. Particular amino acids (within the protein) will have different properties which will dictate the proteins shape and also what it does (likely both relate to reception).
Specific receptors can be tricked by (evolved) viruses (e.g. ACE2 receptors in our respiratory tract can be tricked by S protein on coronavirus so the virus can enter cell)

Question 12

Explain exquisite control:

Answer 12

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 13

What are the two main types of receptors

Answer 13

Intracellular receptors
Membrane-bound/cell surface receptors

Question 14

Describe intracellular receptors:

Answer 14

Ligand/Primary messenger is generally hydrophobic and/or small – lipid soluble, can enter the cell without binding to proteins
Least common method of signalling
- e.g. Testosterone, estrogen, progesterone, thyroid hormones bind to receptors within the cytoplasm and move to nucleus as a complex (many hormones go via this, especially lipid ones)

Question 15

Membrane-bound/cell surface receptors: (focused on)

Answer 15

Primary messenger is generally hydrophilic and/or large
Most common method of signalling
eg. G Protein Coupled Receptor, Receptor Tyrosine Kinase, ligand-gated ion channel

Question 16

Name three example of receptors

Answer 16

GPCR
Ligand gated ion channels/receptors
Extra: Receptor Tyrosine Kinase/RTK (one with rabit ears)

Question 17

Describe a GPCR

Answer 17

Structure:
They are transmembrane proteins - has alpha helices that pass through the PM 7 times (enough hydrophobic residues must be on the protein in the fatty part of PM)
Hundred of different GPCRs exist w/ many different ligands

Functions:
Diverse functions: development, sensory receptors (vision, taste, smell).

Process:
When the signalling molecule is attached to the GPCR, there is a structural change, and the G protein can interact and become active. The G proteins swaps its GDP for GTP - GDP is DISPLACED, NOT phosphorylated
Then the activated G protein dissociates from the receptor/GPCR, and an enzyme is activated to elicit a cellular response.
Then, the G Protein has GTPase activity, promoting its release from enzyme and reverting it back to resting state (GTP loses a phosphate due to enzyme to produce GDP NOT DISPLACEMENT)

Again, G proteins are molecular switches which are either on or off depending on whether GDP or GTP is bound (GDP and GTP are similar to ATP)

GPCR’s are the target for roughly 1/3 of modern drugs

Question 18

Ligand gated ion channels/receptors

Answer 18

Structure:
Channelling receptors that contain a gate for specific ions

Function:
Binding of ligand (e.g. neurotransmitter) at specific site on receptor elicits change in shape of protein (parts of proteins can physically move to open/close gate)
Channel opens and specific ions can pass through (e.g. Na+, k+, Ca2+, and/or Cl-)
When ligand disassociates, gate closes

Examples:
Exists in nervous system: released neurotransmitters such as acetylcholine from vesicles in axon terminals of neurons bind as ligands to ion channel receptors on target cells (e.g. muscle cells) to allow for ions to enter/propagate action potentials

Ion channel/ionotropic receptor = membrane protein through which specific ions can travel, in response to ligand binding

Ion channels may not be in receptor proteins (not 100% sure)

Question 19

Describe a signal Transduction Pathway

Answer 19

Signals relayed from receptors to target molecules via a cascade of molecular interactions

Question 20

Describe a phosphorylation cascade

Answer 20

Signalling molecule binds, receptor changes and eventually (e.g. via activating a prior secondary/relay molecule and therefore producing a second messenger) activates a protein called a kinase. This protein transfers of a phosphate group from ATP to the next protein which typically activates it. This process can continue for a while (if the next proteins are also kinases)
Typically result in cell response

Importantly there are phosphatases that dephosphorylate the kinases (so inactive but recyclable)

Question 21

Protein kinases

Answer 21

Enzymes that transfer a phosphate group from ATP to another protein. Typically this activates the protein, Can also deactivate proteins but either way changes the structure and therefore function

Phosphorylation cascade is when there is a series of protein kinases each adding a phosphate to the next kinase

Question 22

Phosphatases

Answer 22

Enzymes that dephosphorylate (remove the phosphate) rendering the protein inactive but recyclable (available for subsequent activation), as we don’t want it to stay activated. This process is as quick as the cascade, and will keep occurring while signal molecule is still there in most systems

Question 23

What amino acid residues can be phosphorylated

Answer 23

In a given protein, not all amino acid residues can be phosphorylated, and typically serine or threonine residues are phosphorylated, meaning mutations affecting these residues could be detrimental (mean protein doesn’t function).

Question 24

Describe phosphorylation

Answer 24

Addition of a phosphoryl group to molecule.
It is present in cell signalling pathways.
Phosphorylation is not just to do with pathways but many other proteins that need to be phosphorylated to be active/inactive as well

Question 25

Describe second messenger

Answer 25

Second messengers are not proteins but small compounds (typically cyclic AMP/cAMP or calcium ions) that are generated when the enzyme after the receptor becomes activated. It activates downstream kinase proteins which could be the start of the phosphorylation cascade.

All non-protein molecules that act in cell-signalling that aren’t the first molecule are all called second messengers

Question 26

How does cAMP act as a secondary molecule

Answer 26

For example, a cAMP secondary molecule may be produced when a G-protein activates an enzyme (called adenylyl cyclase) which causes ATP to be converted into cAMP. This cAMP activates downstream proteins which could start phosphorylation cascade.

Importantly, this pathway is disrupted by cholera toxin - cholera toxin binds and interferes with enzyme which causes it to be continuously active which gives symptoms (too much cAMP)

Also, cAMP is broken down by phosphodiesterase (PDE) in order to turn off the response

Question 27

How does Ca2+ act as a secondary molecule

Answer 27

Example of CA2+ and IP3 in GPCR signalling:
A GPCR is attached to by ligand
G protein is activated to produce GTP
This then activates an enzyme called phospholipase C
Phospholipase C cleaves membrane bound lipids that are dangling down into the cytosol to create lipid bi-products - IP3 and DAG
The IP3 interacts with the IP3-gated calcium channel to allow for Ca2+ to leave the ER lumen
The calcium therefore acts as a secondary signal to activate various other proteins and cause cellular responses (maybe muscle)

Question 28

Describe the concentration of calcium in different areas of the cell

Answer 28

The calcium concentration in our cytosol is very low (typically ~100nm)
The calcium concentration outside of the cell, in the (smooth) ER and in the matrix is very high (more than 1000-fold higher). Calcium in smooth ER important in muscle contraction.
The extreme differences is what makes it an effective signalling molecule

Maintenance of concentration via calcium pumps is important as high [Ca2+] can damage cells!

There are pumps that actively pump Ca2+ out of cytosol, into the ER, and/or matrix or outside of cell

Question 29

Why are there so many steps involved in signal transduction pathways

Answer 29

Amplifies the response (e.g. 1 molecule of epinephrine can end up producing 10^8 glucose 1-phosphate molecules. The glucagon breaks down into glucose-1-phosphate which is then converted into glucose 6-phosphate which can then be used in glycolysis and make ATP)

Provides multiple control points (multiple points for intervention) - e.g. another bound receptor may stop the steps

Allow for specificity of response (temporal and spatial) despite molecule in common (e.g. sometimes things only happen in one area of the cell)

Allow for coordination with other signalling pathways

Question 30

What can be a cellular response

Answer 30

Gene expression (often it activates a transcription factor which enters nucleus and drives transcription of a gene)

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 31

Why is turning off the response important

Answer 31

For homeostasis

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 cells is ready to respond again if required

E.g. cAMP is broken down by phosphodiesterase (PDE) - there are many types of PDEs in different pathways (e.g. Viagra)

Caffeine blocks the action of PDE (therefore stay wired)

Inhibition of specific PDEs can also be a therapeutic approach (e.g. Viagra - inhibits a specific cGMP - degrading PDE)

Question 32

How does adrenaline act as ligand/signalling molecule

Answer 32

Adrenaline activates GPCR which activates cAMP and two protein kinases in a phosphorylation cascade
This eventually results in many active glycogen phosphorylase which can convert glycogen to glucose 1-phosphate (glycogen breakdown)
Glucose 1-phosphate is then converted into glucose 6-phosphate which can be used in glycolysis for ATP

Due to amplification, one molecule of adrenaline/epinephrine can produce 10^8 glucose 1-phosphate molecules