Lecture 14. Introduction to Cell Signalling Flashcards
What do all cells interact with?
Their environment and with their neighbours
What do cells receive and act on?
Signals from beyond their plasma membrane
What do the membrane proteins in bacteria act as?
Information receptors
What do plant cells respond to?
Variations in sunlight
Growth hormones
Gravity
What do animal cells respond to?
Respond to the metabolic activities of neighbouring cells
Place cells during embryogenesis by recognising developmental signals
Exchange information about ion and glucose concentrations
What do unicellular eukaryotes respond to?
Local environment (food, light, oxygen, etc.)
Mating signals
What is the universal pattern of signalling and responses?
Signal → Receptor → Response
What is a signal?
Information from beyond the plasma membrane
What is a receptor?
Information detector
What is amplification?
Small signals are (usually) amplified within the cell to give a large response
What are response(s)?
Chemical changes and/or changes in gene expression
What are most signals?
Ligands
What are ligands that stimulate pathways called?
Agonists
What are most agonists?
Natural ligands (e.g serotonin)
What are ligands that inhibit pathways called?
Antagonists
What are most antagonists?
Drugs (e.g antihistamines)
What is direct contact signalling?
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
What do gap junctions do?
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
What is autocrine signalling?
The ligand induces a response only in the signalling cell
What happens to autocrine ligands?
Most autocrine ligands are rapidly degraded in the extracellular medium. Often used to enforce developmental decisions
What are eicosanoids?
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)
What is autocrine signalling a common feature of?
Cancers: auto-production of growth hormones stimulates cell proliferation
What is paracrine signalling?
The ligand induces a response in target cells close to the signalling cell
What destroys paracrine ligands?
Destroyed by extracellular enzymes and internalised by adjacent cells
What is the process of paracrine signalling at neuromuscular junctions?
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
What is endocrine signalling?
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)
What are examples of human endocrine tissues?
The pituitary, thyroid and adrenal glands, the pancreas, the ovaries and the testes
What is acetylcholine and what does it do?
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
What two mechanisms provide specificity?
Cell-type specific expression
High affinity interactions
What are the two types of cell-specific expression?
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)
What does a high affinity interaction mean?
There is a precise molecular complementarity between ligand and receptor, mediated by non-covalent forces (like enzyme-substrate and antibody-antigen interactions)
What are signals that are amplified cause?
Enzyme cascades
What is an enzyme cascade?
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.
What do enzyme cascades produce?
Amplifications of several orders of magnitude within milliseconds
When does a pathway become desensitised?
When a signal is presented continuously
When does a system regain sensitivity?
When the signal falls below a threshold level
