Cell Signalling and 2nd Messengers

Question 1

Why may signalling between cells be needed during embryonic development?

Answer 1

• E.g. during development of embryo, cells have to communicate (or signal) to determine specific role and position that each cell will adopt:
  o Cells will move to correct position
  o Cells will die or apoptosis (we don’t have tails!)
  o Cells will differentiate e.g. nerve cells or muscle cells

Question 2

What are the types of intercellular communication (between cells)?

Answer 2

• Autocrine signalling: cells signalling to themselves
  o Ligand or signal binds to receptors on the same cell or group of cells, as seen during early development or cancer/tumour growth (stimulating proliferation)
  o A single signalling cell receives a weak autocrine signal
  o In a group of identical signalling cells, each cell receives a strong autocrine signal

Question 3

What are the four mechanisms of cell-cell communication?

Answer 3

Short range signals:

Contact-dependent
No signals are secreted, and cells make direct contact with each other.
E.g. in embryonic development; cells which are next to each other have to differentiate into different types of cells.

Paracrine
Signals secreted and diffuse short distance.
Signals stay close to signalling cell and bind to receptors which are on cells close by.
E.g. signals regulate inflammation (e.g. cytokines)

Long range signals (normally extracellular):

Neuronal
Messages (electrical) are carried along axons to specialised junctions (synapses).
Synapses are adjacent to the target nerve cell where chemical signals binds to receptors.

Endocrine
Signals (hormones) can be secreted into the bloodstream.
Cells which produce hormones are endocrine cells.
Examples: insulin

Question 4

How is an intracellular signalling pathway activated?

Answer 4

By an extracellular signal molecule then via a series of intracellular signalling proteins

Question 5

What happens when extracellular signals bind to receptors?

Answer 5

• The target cell responds by means of a specific protein called a receptor
• Signal molecules bind specifically to receptors and initiate a response within the target cell
• Each cell type displays a set of receptors that enables it to respond to a corresponding set of signal molecules produced by other cells

Question 6

What are the four cellular responses to signals?

Answer 6

Survival
Divide
Differentiate
Apoptosis/death

Question 7

What is acetylcholine and what is its role as a signaller?

Answer 7

• Acetylcholine is a neurotransmitter (monoamine)
• In cardiac muscle, acetylcholine acts as an inhibitory signal to induce decrease in contraction/bradycardia
• In skeletal muscle, acetylcholine acts an excitatory signal to induce contraction

Question 8

How does acetylcholine trigger membrane hyperpolarisation in cardiac muscle?

Answer 8

Acetylcholine triggers membrane hyperpolarisation in cardiac muscle

• Muscarinic receptors (mAChRs) are G-protein-coupled receptors that activate other ionic channels via a second messenger cascade
• Activation causes hyperpolarisation and a decrease in cardiac activity

Question 9

How does acetylcholine trigger membrane depolarisation in skeletal muscle?

Answer 9

• Nicotinic acetylcholine receptors (nAChR) are non-selective cation ion channels
• Activation causes depolarisation resulting in activation of skeletal muscle contraction
• ACTIONS DEPEND ON RECEPTOR TYPE

Question 10

How does acetylcholine trigger secretion in pancreatic acinar cells?

Answer 10

• Pancreatic acinar cells respond to Ach (acetylcholine) via mAChR (as in cardiac cells) but activate different signalling pathways that lead to secretion of digestive enzymes
• The same signal molecule can bind to identical receptor proteins yet produce very different responses in different types of target cells
• Receptor differences are not always the explanation for the different effects
• This reflects differences in the internal machinery to which the receptors are coupled

Question 11

What is cAMP?

Answer 11

cAMP (cyclic adenosine 3’,5’-monophosphate)

• cAMP was the first second messenger to be identified
• It has a fundamental role in many cellular responses

cAMP stimulates cAMP-dependent protein kinase A (PKA)

• PKA is a protein composed of two types of subunits
• PINK = the catalytic subunit
• BLUE = the regulatory subunit

Question 12

How does calcium function as an intracellular messenger?

Answer 12

• Calcium ions are stored in the endoplasmic reticulum (ER)
• When the cell is activated, calcium ions (Ca2+) enter the cytosol via ion channels either from the extracellular environment or released from the ER

Question 13

What are molecular switches?

Answer 13

• Many intracellular signalling proteins behave like molecular switches:
  o On receipt of a signal, they switch from an inactive to an active state, until another process switches them off
  o It is the gain or loss of phosphate groups that determines whether the protein is active or inactive
  o The switch is thrown in one direction by a protein kinase, which adds one or more phosphate groups to the signalling protein, and in the other direction by a protein phosphatase, which removes the phosphate groups from the protein
• It is estimated that 1/3 of the proteins in a eukaryotic cell are phosphorylated at any given time
• Many of the signalling proteins controlled by phosphorylation are themselves protein kinases, and these are often organised into phosphorylation cascades

Question 14

What are the two types of receptors and what do they cause?

Answer 14

Cell-surface receptors
Intracellular receptors

• Receptor binding causes activation of an intracellular pathway (cell response)

Question 15

What are the three classes of cell-surface receptors?

Answer 15

Ion channel-linked receptor
G-protein linked receptor
Enzyme-linked receptor

Question 16

What are ion-channel-linked receptors?

Answer 16

• Belong to a large family homologous, multipass transmembrane proteins
• This type of signalling is mediated by a small number of neurotransmitters
• Neurotransmitters bind and transiently open or close the ion channel (part of the protein)
• Binding briefly changes the ion permeability of the plasma membrane and thereby the excitability of the postsynaptic cell

Question 17

What is an example of an ion-channel-linked receptor and what does it do?

Answer 17

• Five subunits embedded in the membrane
• Two binding sites for acetylcholine (ACh)
• Binding opens channel and allows sodium to enter the cell
• This leads to muscle contraction

Question 18

What are G-protein-linked receptors?

Answer 18

• Largest family of cell surface receptors
• Transduce signals from hormones, local mediators and neurotransmitters
• Belong to large family of homologous proteins
• The receptors act indirectly to regulate the activity of a separate plasma membrane-bound target protein, which can be either:
  o An enzyme
  o An ion channel

Question 19

What do G-protein-linked receptors do?

Answer 19

• Activates intracellular enzymes (cytosolic and membrane bound)
• Regulates:
  1. Heart rate and force
  2. Contraction and smooth muscles
  3. The release of neurotransmitters

Question 20

What are enzyme-linked receptors and what do they do?

Answer 20

• Enzyme-linked receptors, when activated, either function directly as enzymes or are directly associated with enzymes that they activate
• They are formed by single-pass transmembrane proteins that have their ligand-binding site outside and their catalytic or enzyme-binding site inside the cell
• Enzyme-linked receptors are heterogenous in structure compared with the other two classes
• The great majority are protein kinases (or are associated with protein kinases) – the largest class are “receptor tyrosine kinases (RTK)”
• Majority are growth factor receptors

Question 21

What do receptor tyrosine kinases do?

Answer 21

• Each phosphorylated tyrosine on the receptor acts as a binding site for intracellular proteins
• In order to end the signalling protein tyrosine phosphatases remove the phosphates
• Different tyrosine kinases recruit different signalling molecules
• There are several signalling molecules and pathways that are commonly activated by receptor tyrosine kinases

Question 22

What do receptor-tyrosine-kinases do?

Answer 22

• These kinases phosphorylate each other and initiate downstream signalling
• Fibroblast growth factor binding to their receptors causes the receptors to dimerise and this results in the activation of their protein kinases
• There are TWO major signalling pathways:
  1. Ras G protein and the MAP kinase cascade
  2. Phospholipase C to split PIP₂ into IP₂ and DAG

Question 23

What are intracellular receptor proteins?

Answer 23

• A number of small hydrophobic signal molecules diffuse directly across the plasma membrane of target cells and bind to intracellular receptor proteins
• These signal molecules include steroid hormones, thyroid hormones, retinoids and vitamin D

Question 24

What is the mechanism of action for intracellular receptors?

Answer 24

• Intracellular receptors all bind to specific DNA sequences adjacent to the genes the ligand regulates
• Some receptors (e.g. cortisol) are located primarily in the cytosol and enter the nucleus after ligand binding
• Some receptors (e.g. thyroid and retinoid) are bound to DNA in the nucleus even in the absence of ligand
• In either case, the inactive receptors are bound to inhibitory protein complexes, and ligand binding alters the conformation of the receptor protein, causing the inhibitory complex to dissociate

Question 25

What is the nuclear receptor superfamily?

Answer 25

Each receptor contains a short DNA-binding domain
A receptor protein in its inactive state is bound to inhibitory proteins
The binding of ligand to the receptor causes the ligand-binding domain of the receptor to clamp shut around the ligand and the inhibitory proteins to dissociate
Coactivator proteins to bind to the receptor’s transcription-activating domain, thereby increasing gene transcription

Question 26

What happens when communications break down in diabetes mellitus type 1? (not enough signal)

Answer 26

• Diabetes mellitus is characterised by abnormally high levels of sugar (glucose) in the blood
  o Type 1: Insufficient production of the signal insulin
  o Type 2: Either resistance to the effects of insulin (possibly via insulin receptor) or a defect in insulin secretion
• Insulin receptor is a tyrosine kinase receptor

Question 27

What happens when communications break down in a stroke? (too much signal)

Answer 27

• Blood vessel blockage and reduced blood flow results in the death of brain cells nearby
• Dying cells release large amounts of the neurotransmitter glutamate which is toxic at high concentrations
• Through a process called excitotoxicity, glutamate spreads outside the area of initial damage, killing cells and often leading to widespread brain damage