Cell communication

Question 1

why is cell communication important?

Answer 1

• important for development and homeostasis
• can break down in old age -> has pathology

Question 2

what is the range of cell communication?

Answer 2

• short range
• medium range
• long range

Question 3

what is the conceptual basis of communication characterised by?

Answer 3

• speed of signal
• duration of signal
• receptor mechanism of action

Question 4

what can short-range signals be?

Answer 4

1. contact-dependent
2. synpatic

Question 5

what is contact-dependent signalling?

Answer 5

short-range signalling:
- cell has membrane-bound signalling molecule interacting with a receptor on a neighbouring cell
- 1-to-1 communication

example: notch pathway signalling that spaces R8 photoreceptors in Drosophila eye disc

Question 6

what is synaptic signalling?

Answer 6

short-range signalling:
- neuronal
- neurons may have a very small number of targets

Question 7

what can medium-range signals be?

Answer 7

paracrine signalling
- signalling cell releases an endogenous molecule which acts on neighbouring cells
- autocrine: when the signal acts on cells of the same type as the signalling cells

example: Hedgehog expression stimulates Dpp ligand expression which is secreted by a strip of cells in the middle of the Drosophila wing
- Hh and Dpp are large proteins with post-translational modifications and interact with ECM components before diffusing to act downstream in local cells

Question 8

what can long-range signalling be?

Answer 8

endocrine signalling:
- cell releases endogenous molecule into bloodstream
- slow acting but body-wide/systematic changes

example: adrenaline is produced by the adrenal glands and act via adrenergic receptor to produce a wide range of physiological responses including fight or flight

Question 9

how to cells receive signals?

Answer 9

• extracellular signal molecule binds to a receptor on cell surface
• this triggers intracellular signalling molecules which alter the activity of effector proteins

Question 10

how do cells respond to signalling?

Answer 10

the activity of effector proteins is altered:

1. altered metabolism -> a metabolic enzyme is affected
   - e.g. changes in yeast metabolism triggered by different sugars provided as energy source
2. altered gene expression -> gene regulatory protein is affected
   - e.g. expression of Iroquois family genes in E12 mouse embryo
3. altered shape or movement -> cytoskeletal protein is affected
   - e.g. human neutrophil chasing signal given off by a pathogen

Question 11

Is signalling binary (on/off)?

Answer 11

no:
- there is a quantitative response to a signal
- cells can react in different ways based on the strength of signal received
- there is a qualitatively different response to a signal
- a morphogen gradient may form where responses occur at different thresholds

Question 12

what is the French Flag Model?

Answer 12

• Bicoid mRNA is deposited by the mother in the anterior of the egg and forms a gradient of protein which acts as a morphogen
• This gradient defines the anterior parts of the resulting embryo
• Gene expression downstream of bicoid has sharp boundaries
• Thresholds of signal trigger sharp on/off gene expression

Question 13

how does the speed of a cellular response change?

Answer 13

1. by changing the structure of an existing protein e.g. an ion channel response occurs very quick
2. by changing post-translational modifications e.g. phosphorylation is quite fast
3. by changing proteins via gene expression -> this is a slow process

Question 14

why is signal regulation important?

Answer 14

if it isn’t regulated, it can lead to disease:
1. myeloproliferative neoplasms
- JAK2 V617F is a mutation in the negative regulatory pseudokinase domain of JAK2
- mutation is associated with >95% of polycythaemia vera (blood cancer) patients

2. Chronic myeloid leukaemia (CML)
   - BCR-Abl fusion protein causes sustained ABL tyrosine kinase expression by the BCR promoter region, leading to CML
   - ABL kinase can be inhibited by imatinib

Question 15

at which stages can signalling pathways be regulated?

Answer 15

1. extracellularly via ligand or receptor
2. intracellularly via transduction of the signal
3. via effectors that can feed back into their own pathways

Question 16

what is a positive feedback effector?

Answer 16

• effector is stimulated which causes the induction of the same response at a greater amplitude
• short stimulus causes very fast response that is SUSTAINED

e.g. notch pathway lateral inhibition

Question 17

what is a negative feedback effector?

Answer 17

It the component in a feedback system that causes a change to reverse the situation and return the value to the normal range

e.g. JAK/STAT pathway downregulation by SOCS negative regulator

Question 18

what do fast cellular responses require?

Answer 18

the rapid turnover of an effector - effector must be readily available

Question 19

why is the rapid turnover of an effector a wasteful process? how can it be fixed?

Answer 19

• making and then immediately destroying proteins to achieve rapid turnover is wasteful
• to fix this, the activity of the protein must be altered
• add a post-translational modification to an existing protein that can act as a molecular switch that activates/inactivates protein function

Question 20

what are molecular switches?

Answer 20

monomeric/small GTPases which have enzymatic activity to hydrolyse GTP to GDP
- GTP activates the effector
- GDP inactivates the effector

Question 21

how are GTPase molecular switches regulated?

Answer 21

1. GAP (GTPase Activating Protein) - hydrolyses GTP to GDP
2. GEF (GDP to GTP Exchange Factor) - converts GDP to GTP

modulating GAP and GEF levels can rapidly change the activity of the GTPase molecular switch

Question 22

what are the 2 major groups of protein kinase in eukaryotes?

Answer 22

1. serine/threonine kinase (S/T)
2. Tyrosine kinase (Y)

Question 23

give an example of a receptor tyrosine kinase:

Answer 23

EGF receptor (EGFR) is a receptor tyrosine kinase which recognises the epidermal growth factor (EGF)
- it is possible to generate antibodies that specifically recognise phosphotyrosine (pY)

Question 24

give examples of some kinase inhibitors:

Answer 24

ruxolitinib and imatinib

Question 25

what do kinase inhibitors do?

Answer 25

- normally, ATP is bound within the ATP-binding pocket of S/T or Y kinases kinase inhibitors block the ATP-binding pocket

Question 26

what are the 4 types of kinase inhibitors?

Answer 26

1. Type I inhibitors bind to the active conformation of the kinase with the aspartate residue (white backbone) of the DFG motif pointing into the ATP-binding pocket 2. Type II inhibitors bind and stabilise the inactive conformation of the kinase with the flipped aspartate residue facing outward of the binding pocket 3. Type III inhibitors occupy an allosteric pocket that is adjacent to the ATP-binding pocket but does not overlap with it 4. Type IV inhibitors bind to an allosteric pocket remote from the ATP-binding pocket.

Question 27

what are phosphatases?

Answer 27

- they are key regulators of signalling phosphatase can downregulate JAK/STAT signalling in flies: - Overexpression of ptp61c in vivo is able to reduce the tumour phenotype associated with the fly equivalent of the JAK2 V617F mutation