Cell signalling

Question 1

Signal transduction

Answer 1

The conversion of biotic or abiotic info, often arising outside a cell, into a form inside the cell that can be interpreted and acted up - how it detects environment and responds appropriately. It is important to detect and response to and ever changing environment, otherwise, the organism will die.

Question 2

Abiotic info

Answer 2

pH, light, salinity, hydration

Question 3

Biotic info

Answer 3

Hormones, growth factors and neurotransmitters (extracellular signalling molecules) - social control where cells signal their status to other cells. Where signal transduction is aberrant, life suffers e.g. cancer

Question 4

Prokaryote signalling

Answer 4

Poorly understood e.g. quorum sensing in pseudomonas aerunginosa allowing co-ordinated microbial virulence response - signals when environment is good for growth and starts growth

Question 5

Lower eukaryote signalling

Answer 5

Well studied role of mating factors in saccharoomyces ceqrcisae that signals cells of opposite mating type to stop proliferating in preparation for sexual mating

Question 6

Higher eukaryote signalling

Answer 6

Very well studied

Question 7

How can an extracellular molecule be released from a cell?

Answer 7

Exocytosis or diffusion through the plasma membrane. Molecule may be attached to cell surface and therefore can only enable the cell to signal adjacent cells e.g. Notch and Notch-Ligand signalling in development of mammalian organ systems to generate boundaries

Question 8

What happens when a receptor binds to a signalling molecule (ligand)?

Answer 8

Initiates a response within the target cell.
The hormone/receptor complex recruits a second molecule so that the receptor is activated by dimerisation. Other receptors are activated through conformational changes.

Question 9

Where receptors found?

Answer 9

Most are at the plasma membrane e.g. the b-adrenergic receptor for adrenaline. Receptors for some ligands are intracellular e.g. steroid sex hormones - hydrophobic so free to move

Question 10

Endocrine

Answer 10

Co-ordinates cell behaviour over long distances. Specialised cells secrete ligands (hormones) into the bloodstream or sap to be distributed around the body - slow signalling and dilute at receptor

Question 11

Paracrine

Answer 11

Acts over small distances. Diffusion of ligand is limited by ECM and enzymes

Question 12

Autocrine

Answer 12

Cell produces a ligand that binds to its own receptors. Mechanism is the strongest among groups of cells enabling that group to enter a specific developmental pathway

Question 13

Eicosanoids

Answer 13

Fatty acid derivatives made by cells in all mammalian tissues. On tissue damage, eicosanoid production increases and acts in an autocrine fashion to mediate pain, fever and inflammatory response

Question 14

Contact dependent signalling

Answer 14

Signalling of integral membrane proteins to adjacent cells

Question 15

Combinatorial signalling

Answer 15

Cells of multicellular organisms can be exposed to many signalling molecules in different combinations. The response of the cell depends on the combination.

Question 16

What can a signal instruct a cell to do?

Answer 16

Some signals are required for a cell to survive, or it will undergo apoptosis.
Some instruct the cell to proliferate, others to differentiate

Question 17

In the absence of any signals…

Answer 17

The cell dies. It must receive a minimum complement of signalling molecules.

Question 18

Factors determining the response of a cell to a combination of signalling molecules

Answer 18

The subset of receptors that the cell possesses to detect the signals and the nature of the intracellular machinery by which the cell interprets the signal. e.g. Ach has different effects in heart muscle and salivary glands due to differing Ach receptors.

Question 19

Integrated reactions, Ach and viagra

Answer 19

Signalling requires communication between multiple cell types i.e. signs received by one cell imitates a cascade, impacting other cells. e.g. Ach is released on activation of the sympathetic nervous system, through endothelial cells to SM seeks. NO synthase then takes place due to activation outside the SM cell. It diffuses out, causing rapid relaxation of the SM cell. Also activates cAMP. The key to the speed of this process is the rapid turnover rates of NO and cGMP phosphodiesterase.
Viagra is an inhibitor of cGMP. The half life for cGMP therefore rises and persists in the cell, keeping blood vessels released and increasing blood flow to the penis (viagra stops degradation and cGMP)

Question 20

G-protein linked receptors

Answer 20

Cell surface receptor family with a huge variety of signalling ligands e.g. metabolism, learning and memory.
Common structure of 7 transmembrane passes, extracellular ligand binding domain and intracellular G-protein binding domain = circular arrangement of helices.
~5% human genome

Question 21

‘First messenger’

Answer 21

Ligand for the receptor = first messenger which evolved from non-G-protein linked structures, similar in lower organisms.

Question 22

Where are G-proteins found?

Answer 22

At the cytoplasmic face of a plasma membrane. They are bound to GDP in their inactive state. Alpha and gamma subunits are attached to the plasma membrane.

Question 23

cAMP

Answer 23

Cellular functions are dependent of cAMP e.g. LH targets ovary causing progesterone secretion. G alpha subunits binds to different effector molecules.
Second messenger is cAMP.
It is synthesised by adenyl cyclase (AC) which is activated by one type of G-protein (Gas) and inhibited by Gai. AC synthesises from cAMP from ATP and cAMP and is degraded by cAMP phosphodiesterase to turn off the signal.

Question 24

G-protein regulated adenyl cyclases

Answer 24

Can be regulated by Gbg and the second messenger can be Ca2+ for added complexity. Different hormones bind to the receptor which are either stimulator or inhibitory for AC. The end result is that cAMP production is up regulate doc blocked.

Question 25

cAMP-dependent protein kinase (PKA)

Answer 25

cAMP activates PKA

Question 26

More molecules of cAMP than starting hormone

Answer 26

The extracellular signal is amplified.

Question 27

cAMP binding in inactive PKA

Answer 27

Releases regulatory subunits from the catalytic subunits , therefore, activation the catalytic subunit phosphorylation.

Question 28

What does PKA do?

Answer 28

Phosphorylates other molecules. Has 4 subunits - 2 regulatory and 2 catalytic. The regulatory keep the catalytic from attaching phosphate to things. cAMP binds to regulatory to release catalytic. PKA then activates the cAMP response element binding protein (CREB), which is a common target of PKA and therefore, the transcriptional profile of the cell is changed in response to a hormone. The phosphate groups must be removed from the proteins activated by PKA so that activating signals of not persist longer than necessary. Dephospho rylation of serine and threonine achieves 4 types of str/thr protein phosphatatase - activates transcription of new genes.

Question 29

IP3 and Ach

Answer 29

Cellular functions may also be dependent on IP3 e.g. Ach targets pancreas, causing amylase section. 2nd messenger. Activation of G(alphaO)/G(alphaq) activate effector enzyme phospholipase C, which makes IP3 from PIP2. IP3 goes to where it binds to IP3 receptor (calcium channel)

Question 30

Calcium storage

Answer 30

ER is a cellular storage of calcium. Cell maintains nM cones of Ca2+ and permits release into cytosol to continue IP3 pathway

Question 31

Protein kinase C

Answer 31

Phosphorylates substrates, permitting cell to change its behaviour.

Question 32

Inositol lipids

Answer 32

Found at the plasma membrane - cytosolic leaflet and inositol PI, PIP, PIP2 and IP3

Question 33

Growth factors

Answer 33

Collective name for signalling molecules

Act locally at low concs and respond slowly with many intracellular signalling steps.

Question 34

Receptor tyrosine kinases

Answer 34

Include insulin on the insulin receptor which stimulates carbohydrate utilisation and protein synthesis. Show a common mechanism or activation.

Question 35

Activation mechanism of receptor tyrosine kinases

Answer 35

Extracellular domain of a receptors monomer binds the growth factor ligand. A conformational change on ligand binding permits the receptor dimerisation. 2 receptors phosphorylate each other on tyrosine residues in an ‘activation lip’. Phosphorylation of the lip permits full activation and phosphorylation on a range of other tyrosine residues. Autophosphorylation of the receptor tyrosine kinases imitates a signalling cascade that involves adapter proteins, the small monomeric G-protein Ras ad the MAP protein kinase cascade to transduce the signal to the nucleus. The signal eventually elicits a new round of transc/transl to change cell behaviour as directed by the original ligand.

Question 36

Cell surface receptors not posing an intrinsic tyrosine kinase activity

Answer 36

Have a trusonie linase encoded by another gene which is often non-covalently associated with the receptor. Members of this receptor family include integins which bind the ECM

Question 37

Cytoplasmic tyrosine kinase FAK (Focal Adhesion kinase)

Answer 37

Activated by the activation of integrin matrix binding

Alters cell-metric properties.

Question 38

Cytokine receptors

Answer 38

Activated by a large family of local mediators - cytokines and are very important in immune system signalling.
Cytokine signalling through cytokine receptors depends on the JAK family of cytoplasmic tyrosine kinases. On ligand binding to receptors, homodimer JAKs phosphorylate each other on tyrosine residues. The activated JAKs now phosphorylate the receptor on tyrosine residues, allowing the STAT (signal transducers and activators of transcription) proteins to bind. These are phosphorylated in turn, dimerise, dissociate from the receptor and migrate to the nucleus to initiate transcription. The pattern of transcription is detrained by the original signal and cell type.

Question 39

Transforming growth factor-beta and bone morphogenetic proteins (BMP)

Answer 39

Can act in an endocrine or paracrine fashion. Important in development and regulate pattern formation, proliferation, differentiation etc. They signal through single pass transmembrane proteins with a cytosolic ser/thr-kinase domain and transmit the signal to the nucleus to imitate transcription

Question 40

Receptor guanylyl cyclases

Answer 40

Single transmembrane pass receptors. Synthesise cGMP form GTP. cGMP activates PKG. Many ligands are unidentified for mammalian GC-A and GC-B - natriuretic peptides involved in salt and fluid homeostasis. Atrial natriuretic peptide is an endocrine hormone produced by the heart in response to an increase in blood pressure. It acts on GC-A in the kidney to secrete water and Na+ and relaxes SM, both affecting a decrease in blood pressure. Brain natriuretic peptide has a similar effect on GC-A.

Question 41

Wnt and hedgehog (Hh) recipes

Answer 41

Multi-transmembrane domain spanning receptor but do nit couple to G-proteins. The ligands function primarily in developmental processes e.f. sonic Hh signalling ligand has a patched1 receptor causing L/R asymmetry.

Question 42

Frizzled (Fz) proteins

Answer 42

The 7 transmembrane pass proteins in wnt receptor. Not coupled to G-proteins.

Question 43

Wnt proteins

Answer 43

Wnt proteins play major roles in brain development, limb patterning etc. They are modified with palmitate and so are within the plasma membrane from which they are secreted. Wnt proteins can only mediate contact dependent signalling
Causes change in gene transcription in target cells through TCR transcription factor but TCF only activated transcription in the presence of Beta-cantenin.
In Wnt presence, Fz can interact with its co-receptor LRP. LRP is phosoprhyalted by GSK3 and then auxin can bind to phoph-LRP which disrupts the GSK3-AOC-Axin-beta-cantenin complex. B-cantenin is therefore no longer phosphorylated. B-cantenin accumulates in the cell and translocates to the nucleus.

Question 44

Beta-cantenin

Answer 44

In the absence of wnt, is maintained in a complex with actin, adenomatosis polyposis coli (AOC).
Phosphorylated by GSK3 and is targeted for degradation in the proteasome.
As the b-cantenin is degraded, it does not bind to TCand the elone TCF protein inhibits transcription of Wnt dependent genes.
B-ceantenin associated with TCR to up regulate the expression of wnt dependent genes

Question 45

Hh signalling

Answer 45

Conserved throughout animals and involved in tissue and organ formation. Mutations are in genetics.
Hh is synthesised as a precursor that is then cleaned. The thiol side chain of cyc258 performs a nucleophilic attack on the carbonyl carbon of gly257 to form a thirster. C-terminal catalyses linkage of c-termins to cholesterol and a palmitoyl group added to n-terminys. Occurs intracellularly and Hh is tethered to the plasma membrane.

Question 46

In drosophila, Hh

Answer 46

Ultimately activates transcription of target genes through ci transcription factor in complex with CBP. With no Hh, Ptc represses Smo function and Smo is maintained in internal vesicles.

Question 47

Ci

Answer 47

Maintained in a microtubule bound complex with Fu (a kinase) and Cos2 (a kinesis-like motor protein). Ci is phosphorylated in a process involving PKA, GSK3 and CK1. Phosphorylated Ci is proteolytically degraded by Slimb n a process requiring a proteasome. It represses transcription of hh-resposnive genes. Hh binds to Ptc releoveing its repression Smo.

Question 48

Smo

Answer 48

Moves to the plasma membrane when relieved of repression. It is then phosphorylated by PKA and CK1. Fu and cos2 also become phosphorylated and form a complex with Smo. The complex is then released from microtubules. Ci is not phosphorylated in this complex and translocates to the nucleus where it mediates transcription in complex with CBP>

Question 49

Hh in human

Answer 49

2 ptc genes and 3 gli transcription factors. No co2 ortholog and role of fu othologs is uncertain. Primary cilium may be centre for hh signal transduction..