Lecture 14. Introduction to Cell Signalling

Question 1

What do all cells interact with?

Answer 1

Their environment and with their neighbours

Question 2

What do cells receive and act on?

Answer 2

Signals from beyond their plasma membrane

Question 3

What do the membrane proteins in bacteria act as?

Answer 3

Information receptors

Question 4

What do plant cells respond to?

Answer 4

Variations in sunlight
Growth hormones
Gravity

Question 5

What do animal cells respond to?

Answer 5

Respond to the metabolic activities of neighbouring cells
Place cells during embryogenesis by recognising developmental signals
Exchange information about ion and glucose concentrations

Question 6

What do unicellular eukaryotes respond to?

Answer 6

Local environment (food, light, oxygen, etc.)
Mating signals

Question 7

What is the universal pattern of signalling and responses?

Answer 7

Signal → Receptor → Response

Question 8

What is a signal?

Answer 8

Information from beyond the plasma membrane

Question 9

What is a receptor?

Answer 9

Information detector

Question 10

What is amplification?

Answer 10

Small signals are (usually) amplified within the cell to give a large response

Question 11

What are response(s)?

Answer 11

Chemical changes and/or changes in gene expression

Question 12

What are most signals?

Answer 12

Ligands

Question 13

What are ligands that stimulate pathways called?

Answer 13

Agonists

Question 14

What are most agonists?

Answer 14

Natural ligands (e.g serotonin)

Question 15

What are ligands that inhibit pathways called?

Answer 15

Antagonists

Question 16

What are most antagonists?

Answer 16

Drugs (e.g antihistamines)

Question 17

What is direct contact signalling?

Answer 17

A protein (ligand) on the signalling cell binds a protein (receptor) on the target cell. The target cell responds.
Such signalling is common in tissue development. e.g, cell-to-cell contact controls eye development in Drosophila

Question 18

What do gap junctions do?

Answer 18

Exchange small (< 1.2 kDa) signalling molecules and ions, co-ordinating metabolic reactions between cells.
e.g., gap junctions are made and broken during embryo development
e.g., electrical synapses use gap junctions between neurons for rapid electrical transmission

Question 19

What is autocrine signalling?

Answer 19

The ligand induces a response only in the signalling cell

Question 20

What happens to autocrine ligands?

Answer 20

Most autocrine ligands are rapidly degraded in the extracellular medium. Often used to enforce developmental decisions

Question 21

What are eicosanoids?

Answer 21

Autocrine ligands derived from fatty acids and exert complex control
e.g aggregation of platelets in the immune system, integration of pain and inflammatory responses (aspirin is an antagonist), and contraction of smooth muscle (uterus)

Question 22

What is autocrine signalling a common feature of?

Answer 22

Cancers: auto-production of growth hormones stimulates cell proliferation

Question 23

What is paracrine signalling?

Answer 23

The ligand induces a response in target cells close to the signalling cell

Question 24

What destroys paracrine ligands?

Answer 24

Destroyed by extracellular enzymes and internalised by adjacent cells

Question 25

What is the process of paracrine signalling at neuromuscular junctions?

Answer 25

A nerve impulse stimulates movement of synaptic vesicles, which fuse with the cell membrane releasing acetylcholine
Acetylcholine stimulates channel opening, allowing ion exchange
The muscle twitches, and acetylcholinesterase degrades the acetylcholine

Question 26

What is endocrine signalling?

Answer 26

The ligand is produced by endocrine cells and is carried in the blood, inducing a response in distant target cells. The ligands are often called hormones (from Gk: to set in motion)

Question 27

What are examples of human endocrine tissues?

Answer 27

The pituitary, thyroid and adrenal glands, the pancreas, the ovaries and the testes

Question 28

What is acetylcholine and what does it do?

Answer 28

Acetylcholine as a neurotransmitter is released into a neuromuscular junction and signals in a paracrine manner
As a hormone it signals in an endocrine manner

Question 29

What two mechanisms provide specificity?

Answer 29

Cell-type specific expression
High affinity interactions

Question 30

What are the two types of cell-specific expression?

Answer 30

Certain receptors are only present on certain cells
(e.g hyrotropin-releasing hormone (TRH) triggers pituitary responses but not liver responses)
Molecules downstream of the receptor are only present in some cells
(e.g epinephrine (adrenaline) alters glycogen metabolism in hepatocytes but not in erythrocytes)

Question 31

What does a high affinity interaction mean?

Answer 31

There is a precise molecular complementarity between ligand and receptor, mediated by non-covalent forces (like enzyme-substrate and antibody-antigen interactions)

Question 32

What are signals that are amplified cause?

Answer 32

Enzyme cascades

Question 33

What is an enzyme cascade?

Answer 33

The receptor or an enzyme associated with the receptor is activated
This now catalyses the activation of a second enzyme, which activate multiple molecules of a third enzyme, etc.

Question 34

What do enzyme cascades produce?

Answer 34

Amplifications of several orders of magnitude within milliseconds

Question 35

When does a pathway become desensitised?

Answer 35

When a signal is presented continuously

Question 36

When does a system regain sensitivity?

Answer 36

When the signal falls below a threshold level