Signal Transduction (14-19) Flashcards
Do cells live in isolation?
No
→ all cells interact with their environment and neighbours
→ receive and act on signals from beyond their plasma membrane
signalling controls all aspects of our behaviour: growth, differentiation and development, metabolism
Do bacteria signal?
Yes - they have membrane proteins that act as information receptors
signals: pH, osmotic strength, food, oxygen, light, noxious chemicals, predators, competitors
What is the universal signalling pathway?
single → receptor → response
What does each stage of signal transduction mean?
Signal → information from beyond the plasma membrane
Receptor → information detector
Amplification → small signals are (usually) amplifies within the cell to give a large response
Response(s) → chemical changes and/or changes in gene expression
→ you can’t just turn a system on - feedback pathways are required
What are ligands?
A chemical messenger produced by signalling cells that bind to receptors in or on target cells
agonists → ligands that stimulate pathways - most natural ligands (e.g. serotonin)
antagonists → ligands that inhibit pathways - most drugs (e.g. antihistamines)
What is direct contact (type of signalling)?
A protein (ligand) on the signalling cell binds to a protein (receptor) on the target cell
→ target cell response
→ common in tissue development (e.g. cell-cell contact controls eye development in Drosophila)
What is gap junction (type of signalling)?
Gap junctions: exchange small signalling molecules and ions, co-ordinating metabolic reactions between cells
→ ability of cells to inform other cells of intracellular content
→ gap junctions are made and broken during embryo development
→ electrical synapses use gap junctions between neurones for rapid transmission
What do gap junctions do in electrical synapses?
Clusters of gap junctions connect the interior of two adjacent neurones
→ enable the passage of electrical current carried by ions + intracellular messengers + small metabolites
(connections between pre- and post- synaptic membranes)
What is autocrine signalling?
The ligand induces a response only in the signalling cell (self stimulation)
→ signalling cell expresses ligand and receptors
→ most autocrine ligands rapidly degrade in extracellular medium
e.g. eicosanoids: autocrine ligands derived from fatty acids and exert complex control (aggregation of platelets in immune system, integration of pain/inflammatory response, contraction of smooth muscle)
also 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
→ architecture of cells limits ability of ligand to diffuse
→ destroyed by extracellular enzymes and internalised by adjacent cells
e.g. paracrine signalling occurs at neuromuscular junctions
What occurs during paracrine signalling at neuromuscular junctions?
- a nerve impulse is transmitted
- stimulates movement of synaptic vesicles, which fuse with the cell membrane
- acetylcholine is released
- acetylcholine stimulates channel opening, allowing ion exchange
- the muscle twitches, acetylcholinesterase degrades acetylcholine
(response to receptor binding → opening of ion channels)
What is endocrine signalling?
The ligand is produces by endocrine cells and is carried in the blood, inducing a response in distant target cells (ligands often called hormones)
→ human endocrine tissues: pituitary, thyroid and adrenal glands, pancreas, ovaries, testes
Is the distinction of signals always absolute?
No → ligands can belong in more then one class
e.g. acetylcholine → neurotransmitter in neuromuscular junction - paracrine manner, as a hormone - endocrine manner
How is signalling specificity provided through cell-type specific expression?
a. Certain receptors are only present on certain cells
b. Molecules downstream of the receptor are only present in some cells
How is signalling specificity provided through high-affinity interactions?
There is a precise molecular complementarity between ligand and receptor, mediated by non-covalent forces
(affinity: ability of a molecule to find and interact with another molecule)
How is the rate of receptor ligand interactions determined?
R (receptor) + L (ligand) → RL (receptor-ligand couples)
association rate → 2 reactants so defined by second order, conc. of both reactants = k+[R][L]
units = M^-1s^-1
dissociation rate → 1 reactant so defined by first order rate, conc. of the one reactant considered = k-[RL]
units = s^-1
At equilibrium what happens to the rate of association and dissociation?
They are equal
k+[R][L] = k-[RL] → k+/k- = [RL]/[R][L]
Keq = k+/k- = [RL]/[R][L]
units = M^-1
How can the ‘dissociation equilibrium constant’ (Kd) be described?
The reciprocal of Keq →
Kd = k-/k+ = [R][L]/[RL]
units = M
→ favoured by biologists due to familiar units (affinity can be described in terms of conc.)
What is the meaning of low/high affinity?
High affinity → specific interactions
e.g. sptretavidin (Kd ~ 10^-14M for biotin) can scavenge biotin when its very low in conc. - strongest non-covalent bond in nature
Low affinity → less specific interactions
e.g. bovine (Kd ~ 10^-6M for plastic) will interact non-specifically with most things - ideal blocking agent
Is binding just two molecules sticking together and remaining together?
No → binding is a dynamic process - a mixture of association and dissociation
A + B ⇌ AB
off ⇌ on
→ higher affinity = longer time spent together
How are signals amplified?
Enzyme cascades
→ produce amplifications of several orders of magnitude within milliseconds
What is signalling desensitisation?
When a signal is present continuously the signal transduction pathway becomes desensitised
→ when the signal falls below a threshold system regains sensitivity
e.g. walk from bright sunlight into a dark room
What is signalling cross-talk?
Many signalling pathways share common components leading to potential cross-talk
What is signalling integration?
If multiple signals are given, the cell produces a unified response
+ response % - response → net response depends on the integrated output of both receptors
combination of cross-talk and integration → signal responses can be very complex
What are the classes of receptors?
Receptors with intrinsic enzyme activity
Receptors linked to protein kinases
Receptors coupled to target proteins via a G protein
Intracellular receptors
Receptors that are ion channels
What are receptors with intrinsic enzyme activity?
Some receptors are enzymes - binding of ligand activated the enzyme activity
→ prototype for this group is the insulin receptor
What hormones regulate blood glucose levels?
Following intake and ingestion of food, blood glucose can rise dramatically
(pancreatic)
Insulin → lowers blood sugar levels
Glucagon → raises blood sugar levels
(adrenal)
Epinephrine → raises blood sugar levels
Cortisol → raises blood sugar levels
Where are insulin and glucagon produced?
In the pancreas
→ acing cells have digestive function
→ the islets of Langerhans secrete hormones
α cells: glucagon
β cells: insulin
δ cells: somatostatin
How is the insulin receptor made?
IR is made as a single protein, from a single gene
→ following translation the subunits
1. enter ER membrane
2. associate into dimers
3. exported to cell surface, via Golgi
4. during intracellular transport: processed by cleavage into α and β subunit
5. at plasma membrane - displayed as trans-membrane proteins
How does insulin activate the insulin receptor (IR)?
Insulin signalling starts at the plasma membrane
→ insulin binding stimulates allosteric change in IR
→ brings cytosolic domains close
→ leads to auto-phosphorylation
→ results in activation
What is a first/primary messenger/ligand?
An extraceullar substance that binds to a cell-surface receptor and initiates signal transduction that results in a change in intracellular activity
receptor → a protein that binds and responds to the first messenger
What is the first step in insulin signalling?
Activated IR phosphorylates and activates the insulin receptor substrate-1 (IRS-1)
→ signal has been transduced from the extracellular side of membrane to the intracellular side + has been transferred to a soluble protein in the cytosol
What does activated IRS-1 do?
Activated IRS-1 is bound by adaptor molecules Grb2 and Sos
→ signal transferred to Sos - a guanine nucleotide exchange factor (GEF)
What does Sos do?
Sos converts (GDP-bound) Ras to active (GTP-bound) Ras
→ activates Ras
What does activated Ras do?
Activated Ras recruits Raf kinase to the membrane - activates its protein kinase activity
→ Raf phosphorylates and activates MEK kinase
→ MEK kinase activates mitogen-activated protein kinase (MAPK)
→ signal amplified across the cytosol through a MAPK cascade
What does ERK (MAP kinase) do?
Migrates to the nucleus
→ alters gene expression modulating expression of ~100 insulin responsive genes + cyclins/CDKs required for cell division
→ insulin is a growth factor
How is the insulin pathway linked to the epidermal growth factor (EGF) signalling pathway?
Grb2 and Sos are common to both EGF and insulin signalling
→ activation of EGFR and IR recruits the same MAPK cascade - the same genes are modulated in the downstream response
How is insulin receptor substrate-1 (IRS-1) bi-functional?
It binds to Grb2 → MAPK cascade - gene expression changes (Ras dependant)
+
recruits and activates phosphoinositide 3-kinase (PI-3K) to the cytosolic face of the plasma membrane → glucose regulation (Ras independent)
→ growth and glucose metabolism co-ordinated via insulin signalling - not much point growing without food supply
What does PI-3K do?
Phosphorylates the membrane lipid PIP2 to produce PIP3
→ PIP3 is a second messenger
second messenger: never a protein, a small metabolically unique molecule whose conc can change rapidly - relay signals from receptors to target molecules in the cytoplasm or nucleus
What does PIP3 recruit?
PDK1 (PIP3-dependant protein kinase)
→ activates protein kinase B (PKB) aka Akt
What are the 2 responses to insulin?
Growth factor:
Phosphorylation IRS-1 (amplifies) → Grb2 + Sos adaptors recruit Ras → Signal transduction via amplifying MAPK cascade → gene expression changes
Glucose regulator:
Phosphorylation IRS-1 (amplifies) → signal propagation + amplification - conversation of membrane lipids → amplification - lipid dependant kinase activation of PKB → up regulation of glucose entry into cells + glycogen production
What are the cellular responses to insulin?
Within minutes → increased uptake of glucose into muscle cells and adipocytes + altered glucose metabolism by modulation of enzyme activities
→ don’t require new protein synthesis, occur low insulin levels ~10^9 - 10^10M
Within hours → increased expression of: liver enzymes that synthesise glycogen, adipocyte enzymes that synthesis triacylglycerols, genes involved in mitogenesis in some cell lines
→ require continuous exposure to ~10^8M insulin
How is the Ras-independent insulin signalling pathway terminated?
A PIP3-specific phosphatase (PTEN) removes the phosphate at the 3 position of PIP3 - converting it into PIP2
→ PDKI and PKB can no longer be recruited to the plasma membrane, shutting of signalling through PKB
What is diabetes mellitus?
When insulin signalling goes wrong
Type I → Insulin dependent diabetes mellitus
→ deficiency in insulin production, early onset, responds to insulin injection
Type II → Non-insulin dependent diabetes mellitus
→ failure to respond to insulin, typically late onset, associated with obesity
symptoms of both: excessive thirst, frequent urination (polyuria), excretion of large amounts of glucose in the urine (glycosuria)
results in high blood sugar levels
What does activated PKB do?
(in muscle and adipose tissues)
Activated PKB stimulates the movement of the glucose transporter GLUT4 from internal membrane vesicles to the PM
→ increasing uptake of gluocose
also mediates the conversion of excess glucose into glycogen (in the liver/muscles) and to triacylglycerols (in adipose tissue)
