Cell Communication

Question 1

Give an overview of cell communication?

Answer 1

Communication between cells is often mediated by extracellular signal molecules
E.g. proteins, amino acids, nucleotides, steroids, fatty acid derivatives
Nitric oxide and steroid hormones activate intracellular receptors

Reception of these signals depends on a receptor - which induces a conformational change and will eventually initiate a response in the target cell
Most are transmembrane proteins whilst few are intracellular, requiring the signal to pass through the membrane for activation

Simple pathway: extracellular signal molecule, receptor protein, intracellular signalling proteins, effector proteins
Outcome - altered metabolism, gene expression, change in shape/movement, differentiation, division, apoptosis etc…
The effect is dependent on the intracellular pathway activated as some extracellular signals induce different pathways depending on the cell

Question 2

Describe cell communication in single celled organisms?

Answer 2

Interact with their external environment - senses toxins and bind nutrients

Responding to chemical signals secreted by neighbours = ‘quorum sensing’ in bacteria
This leads to sporulation, motility, sexual conjugation and antibiotic production

Yeast mating factor - stops proliferation and induces mating were fusion of 2 haploid cells forms a diploid cell

Question 3

Describe cell communication in multicellular organisms?

Answer 3

Allows communication with and sensing of the extracellular environment
They emit and receive signals
Leads to spatial patterning, allows distal communication, cell proliferation, apoptosis, differentiation, cell guidance and migration etc.

Question 4

How can we categorise types of receptors?

Answer 4

Cell specific - contact dependent (physically touching) and synaptic
Cell type specific - paracrine and endocrine

Autocrine secretion - a cell is signalling to itself
A messenger molecule is released by a cell and is bound to a receptor on the same cell
Paracrine secretion - a cell is signalling to a cell within its surrounding locality
Endocrine secretion - a cell is signalling to a cell to another cell in a completely different organ/system for example

Gap junctions (narrow water filled channels) also allow neighbouring cells to share signalling information i.e. Small intracellular mediators such as Ca2+ or cAMP

Question 5

Describe the intracellular signals?

Answer 5

Small intracellular mediators or secondary messengers activate a cascade of proteins to complete the pathway
They can function as molecular switches - being induced by phosphorylation/GTP binding
They contain small interaction domains - the motif induces proximity, triggering the assembly of a signalling complex in specific combinations

Often made up of feedback loops
Positive - output stimulate production
Negative - output inhibits production

Question 6

How do cells adapt to a stimulus?

Answer 6

Adaptation allows cells to respond to changes in concentration of an extracellular signal molecule - over a wide range of signal concentrations
As oppose to an absolute concentration of the signal
Happens as a result of a negative feedback loop with a short delay

Question 7

How do cells densensitise to a stimulus?

Answer 7

Can result from inactivation of the receptors
Destruction of the receptor via endocytosis into lysosomes = receptor down regulation
They can be deactivated on the cell surface by phosphorylation/methylation
An inhibitory protein can block the signal transduction process
Inactivated the intracellular signalling protein

Question 8

What is are agonists and antagonists?

Answer 8

Ligand - binding partner for receptor
Agonist - molecule that binds a receptor and causes a cellular response
Antagonists - molecules that blocks the response of the ligand

Question 9

What are some receptor families?

Answer 9

G protein-coupled receptors 
Enzyme-coupled receptors 
Adhesion receptors 
Pathogen recognition receptors, toll-like receptors, lectins, antibody receptors 
Ion-channel coupled receptors (nerves)

Question 10

Describe G-proteins receptors?

Answer 10

Heterotrimeric G-proteins with 3 subunits: alpha, beta and gamma
They are soluble cytoplasmic proteins - anchored to membrane via fatty acids (during post translational modification)
The G-protein coupled receptors contain 7 transmembrane helices

Question 11

How is the G-protein swithced between active and inactive forms?

Answer 11

Normally in an inactive form but when a G-protien coupled receptor is activated the G-protein binds GTP
There is an exchange of GDP to GTP on one of the alpha subunits of the trimer, dissociating into 3 monomers
Eventually GTP hydrolysis leads to switching the protein back to the inactive form
This pathway is therefore on a timer as the protein is only active for as long as it takes for GTP to be hydrolysed

The proteins are aided by GAPs (exchange factors) - they bind to the G-protein and help in driving the reaction

Question 12

What are the three major components of the signal transduction system of G-proteins?

Answer 12

G-protein-coupled receptors (GPCRs) - they bind a corresponding agonist extracellularly, inducing a confomational change intracellularly
Heterotrimeric G proteins -  anchored to the cytoplasmic side and activated by GPCRs
Adenylate cyclase (AC) - a transmembrane enzyme that is activated/inhibited by activated heterotrimeric G proteins

Question 13

Describe disease associated GPCR?

Answer 13

Parathyroid hormone receptor (PTHR) expression in the kidney regulates of calcium and phosphorus concentration
Also regulates chondrocyte growth and differentiation

Gene mutation in PTHR causes constitutive receptor activation
Leads to Jansen’s metaphyseal chondrodysplasia (shortlimbed dwarfism)

Question 14

Describe GPCR in autoimmune disease?

Answer 14

Autoimmune disease via antibody agonists/antagonists

Grave’s disease causes hyperthyroidism via thyroid stimulating hormone receptor agonistic autoantibodies
Causes excess generation of cAMP via Gas subunit, weight loss, goitre etc

Hashimoto’s disease caused by antagonistic autoantibodies result in ↓ cAMP, weight gain, fatigue etc, often post partum

Question 15

Describe some toxin activation of GPCRs?

Answer 15

Many pathogens utilise host receptors for their own ends for: adhesion, cell entry, immune subversion or dissemination

Cholera toxin - uses ganglioside GM1 as an entry receptor in to cells
After processing and activation it catalyses ADP-ribose transfer from NAD+ to an arginine residue to Gas (ADP-ribosylation)
Prevents hydrolysis of GTP and increase in cAMP
Increase in Cl- release in to the gut decrease Na+ resulting in watery diarrhoea /death

Pertussis toxin (whooping cough)
This prevents activation of Gai via ADP-ribosylation (↑ cAMP)
Affect ion flux in lung epithelial cells
Life threatening for neonates

Question 16

What are some non-receptor defects that can cause disease?

Answer 16

McCune–Albright syndrome activating Gsa mutations (GNAS gene)- Gain of Function
Two major mutations
Arg201 is in the GDP/GTP binding domain of the protein (Cholera toxin binding site)
Gln227 required for intrinsic GTPase activity

Non germline (somatic) defects can give mosaic/sporadic phenotype  
Café-au-lait phenotype
Increases in melanocyte-stimulating hormone–Gs-cAMP pathway and therefore stimulates melanin pigment production
Fibrous dysplasia osteoblast dysfunction

Question 17

Describe tyrosine kinase receptors?

Answer 17

They have to interact with the extracellular environment because their ligands are hydrophilic - therefore they need a receptor
TKR are transmembrane proteins but they communicate throughout the cytoplasm
When they bind to their ligand they become dimeric = active = inducing trans-autophosphorylation (this is ligand induced dimerization)
The active form of the receptor communicates with the intracellular environment and switches on pathways

Question 18

How is the tyrosine kinase receptor activated?

Answer 18

Trans-autophosphorylation

Three tyrosine residues are phosphorylated and the activation loop changes its conformation - the N-terminal lobe undergoes nearly 21° rotation relative to the C-terminal lobe
This forms part of the substrate recognition site

Question 19

What happens with the TKR after activation?

Answer 19

The intracellular domain of the RTK, when the tyrosine residues become phosphorylated they serve as docking ports for many proteins in the cytoplasm
The proteins in the cytoplasm recognises the phosphotyrosine residues because the proteins have a small domain structure within their structure = Src Homology 2 (SH2) domain

Question 20

What is the role of Src Homology 2 (SH2)?

Answer 20

Small globular domain found in many proteins
It specifically recognises phosphotyrosine, and has a high affinity for them
Once the receptor is active it then recruits many proteins to it, to recognise the receptor
This forms multi-protein complexes at the receptor
These complexes transmit discrete signals to different pathways within the cell

Question 21

Give and example of a TKR and an associated disease?

Answer 21

Insulin receptor - increase glucose uptake

Type-I - lack of Insulin (autoimmune destruction of beta-islet cells)
Type-II - Insulin resistance: complex metabolic disorder however maybe be caused by down regulation of insulin receptors

Mechanism of receptor desensitisation include:
Endocytosis and degradation of receptor (clathrin and clathrin-independent pathways)
Phosphotyrosine phosphatases (PTPases)  
Also down regulation of PI3K and IRS proteins (both downstream signalling components) by high glucose/free fatty acids

Question 22

Give an overview of adhesion receptors?

Answer 22

All multicellular organisms required adhesion receptors
Allows spatial patterning, migration, differentiation, guidance, morphogenesis etc
Cell-cell, cell-matrix interactions

Some mediate homotypic interactions, some heterotypic
A binding to A - A binding onto B

Question 23

What are the types of adhesion receptors?

Answer 23

Ig superfamily - NCAM, VCAM, ICAM, MAdCAM
Cadherin - E-Cadherins, N-Cadherins, VE-Cadherin, flamingo
Integrins - CD11a,b,c/CD18 - other ab dimers often recognise RGD motif in ligand
Selectins - L, E, P - they bind sialyl-LewisX glycosylated proteins
Proteoglycans - contain protein core and GAG side chains

Question 24

Give an example of adhesion-cascade?

Answer 24

Leukocyte adhesion-cascade:
This uses many cells and molecules - the movement of white blood cells

Capture of the leukocyte on the endothelial cell
Selectins- bind sialyl-LewisX sugars, found on eg PSLG-, low affinity, transient in nature
They signal via phosphoinositide-3-kinase(gamma) (PIK(gamma)) resulting in slow rolling along the endothelial layer

Activation through many molecules
Arrest - leukocyte stops rolling (mediated by integrins)

Adhesion is then strengthened by spreading
Then intravascular crawling where paracellular and transcellular transmigration takes place
(leukocyte moves through the endothlial layer)

Question 25

Describe adhesive cell interactions during development?

Answer 25

Embryonic processes involving cell adhesion
Segregation of tissues - during neural tube formation resulting from different expression of adhesion molecules
Dispersion of cells - from a solid tissue resulting from a decrease in cell–cell adhesion
Cell guidance - migration of cells along adhesive guidance cues
Cavity formation - requires a combination of intercellular sealing by tight junctions and vectoral ion and water transport
Cell–cell communication through gap junctions (barrels) is dependent on cell–cell adhesion

Question 26

Describe cell receptors in metastasis?

Answer 26

Metastasis- movement or spreading of cancer cells from one organ or tissue to another
Usually via the bloodstream or the lymph system
Requires Epithelial to Mesenchymal Transition (EMT) plus a MET
Metastasis involves a whole range of adhesion molecules, signalling production of MMP, chemotaxis

Question 27

Describe microbial receptors? example?

Answer 27

Receptors of the immune system need to be able to recognise self and non-self
Metazoans have evolved receptors to recognise microbial motifs (Pathogen Associated Molecular Patterns)

Surface Pattern Recognition receptors
Recognise various PAMPs
Major classes - Toll receptors, carbohydrate binding lectins
Able to signal to produce appropriate responses and production of various cytokines and immune regulators

Question 28

How do pathogens exploit host receptors?

Answer 28

Many essential host receptors are exploited by pathogens

Example: HIV
CD4 on T cells normally interacts with MHCII on APCs leading to activation of T-helper cells
Binding allows previously buried hydrophobic fusion protein to insert in to the membrane
Gp120 is attached to viral membrane via gp41

Question 29

What is a defect in microbial recognition?

Answer 29

Toll mutant Drosophila
Single nucleotide polymorphism in the promoter of the human Dectin-1 promoter increases susceptibility to Candida infection

Question 30

Example of a pathogen exploiting a receptor?

Answer 30

Listeria
Listeriolysin O - the formation of these pores by listeriolysin O in the phagosomal membrane allows access to the phagosomal membrane of two different Listeria-encoded C-type phospholipases that cause the disruption of the phagosomal membrane
Once in the cytosol, the bacteria are able to grow and divide