Week 16 Flashcards
Why are signalling systems needed?
Coordinate activities of cells/tissues in a multi-cellular organism.
What are different examples of signalling systems?
Neurotransmission
Coordination of developmental processes
Homeostasis (maintenance of a constant internal balance)
What are the three types of signalling between cells?
- Free diffusion between cells
- Via cytoplasmic connections
- Direct cell-to-cell contact
Signalling between cells by free diffusion is classified into what three types, according to the distances involved?
Autocrine: signalling and reception by same cell
Paracrine: Signalling between nearby cells
Endocrine: Signalling between distance cells (by ‘hormones’)
(and synaptic signalling)
What is autocrine signalling?
(Signalling by free diffusion)
- Cell secretes chemicals that modify its own behaviour
- Often associated with growth regulation (+ive or -iv)
What is paracrine signalling?
(Signalling by free diffusion)
- Cell signalling to neighbouring cells
- Effects are local and short-lived (degraded easily)
- Important (eg) in actions of neighbouring cells in embryonic development
What is synaptic signalling?
(Signalling by free diffusion)
Highly specific and localised type of signalling between two nerve cells or between a nerve cell and a muscle cell
What is endocrine signalling?
(Signalling by free diffusion)
Ductless glands called endocrine glands secrete hormones into extracellular spaces, which then diffuse into the circulatory system:
Eg: Pituitary, Adrenal, Thyroid glands
Signalling between cells: explain signalling via cytoplasmic connections?
Can you Give an example?
- Transfer of signal from one cell to its neighbour through pores in the membrane (usually gap junctions)
- The fastest mode of cell-cell communication (cells have to be directly touching each other)
e.g. muscle cells in the heart communicate with each other via gap junctions, allowing all heart cells to contract almost simultaneously.
Signalling between cells: explain signalling by cell-to-cell contact?
When would this be important?
- Involves specific interactions between surface molecules on one cell and receptors on another cell
- Responsible for cell-cell recognition in animals
- Important in embryonic development and immune response
What are the two types of signalling molecules?
Local regulators - act on cells in the vicinity (autocrine and paracrine signalling)
Hormones - act at distance (endocrine signalling)
Example of a local regulator (signalling molecules) type is growth factors, what are they?
Can you give an example of this type?
Peptides or proteins that stimulate cell proliferation, may have >1 target cells, hence >1 function
e.g. Nerve Growth Factor (NGF) – a small protein that regulates growth of target neurons
Example of a local regulator (signalling molecules) type is Gases, what are they?
Nitric oxide (N=O) acts as a paracrine signal molecule (transient - half-life is only 1-5sec)
Synthesised from arginine by nitric oxide synthase.
Induces vasodilation in cardiovascular system (Furchgott, Ignarro and Murad were awarded the Nobel Prize in 1998 for discovering this)
Example of a local regulator (signalling molecules) type is Prostaglandins, what are they?
What are their multiple functions?
Modified fatty acids
Multiple roles include:
- Excitability of the uterine wall during childbirth – placental secretion helps to induce labour
- Induction of fever and inflammation in the immune system (pain-alleviating effects of aspirin and ibuprofen are due to their inhibition of prostaglandin synthesis)
Example of a local regulator (signalling molecules) type is Neurotransmitters, what are the different types? Can you give examples? What are their roles?
Acetylcholine
Biogenic amines (e.g. serotonin)
Amino acids (e.g. Glutamate)
Neuropeptides (e.g. endorphins)
- Some neurotransmitters are inhibitory, some are excitatory, and some can be either
- Some occur in both the central nervous system and the peripheral nervous system
What are hormones (signalling molecules)?
Secreted by endocrine glands and transported in bloodstream.
Hormone production controlled by neuroendocrine system (hypothalamus = control centre, commands pituitary which sends signals to hormone producing glands)
How is a negative feedback loop triggered as a result of the endocrine pathway?
A change in some internal or external variable – the stimulus – causes the endocrine cell to secrete a hormone
Upon reaching its target cell via the bloodstream, the hormone binds to its receptor, triggering signal transduction that results in a specific response.
Example of a negative feedback loop? (endocrine pathway)
Secretin signalling is an example of a simple endocrine pathway.
- Stimulus: acid secretion into duodenum
- Stimulates S cells to secrete secretin
- Via the bloodstream, secretin reaches the pancreas and target cells release bicarbonate, which raises the pH in the duodenum
- Negative feedback – response reduces the stimulus
(Another example would be temperature maintenance in the body, as is oscillates around a set point - homeostasis)
What is homeostasis?
maintenance of a relatively stable internal environment in the face of stress from the external and internal environments
Examples of homeostasis in the human body?
-Body temperature: ~37oC
-Blood pH: ~ 7.4
-Arterial blood pressure: ~120/80 mm Hg
-Blood glucose 90 mg/100 ml
Our internal environment isn’t constant its in a dynamic equilibrium where…
changes are kept within an acceptable range (by means of negative feedback loops)
What are the two main classes of hormones? Give examples?
- Peptides and proteins eg insulin
- Steroids eg testosterone
How do peptide and protein hormone classes work?
Bind to receptors on the cell surface,
Trigger events within cell cytoplasm through second messengers
How do steroid hormone classes work?
Manufactured from cholesterol
Can pass across lipid bilayer of plasma membrane and bind to receptors within cell
What is signal transduction?
The conversion of a signal at the cell surface to a specific cellular response is a multi-step process termed a signal transduction pathway
Briefly what is the first main stage of signal transduction?
- Reception of the signal at the cell surface
Involves binding of a signal molecule to a specific receptor at the cell surface, changing the conformation of the receptor…
Briefly what is the second main stage of signal transduction?
- Transduction of the signal
Converting the signal into a response usually involves multiple steps
(e.g. protein phosphorylation by protein kinases)
Advantage of multistep pathways in transduction of the signal in signal transduction?
- Multistep pathways can amplify a signal: A few molecules can produce a large cellular response
- Multistep pathways provide more opportunities for coordination and regulation
Briefly what is the third main stage of signal transduction?
- Cellular response - the ‘output response’
Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities
Where may response occur in the third main step of signal transduction?
- The response may occur in the cytoplasm or may involve action in the nucleus
- Some pathways regulate the activity of enzymes
- Other pathways switch on genes by activating transcription factors
How can more than one response occur in signal transduction?
Due to the reception of a single ligand
Explain why signal transduction has signal specificity?
- Different kinds of cells have different collections of proteins
- Differences in proteins give each kind of cell specificity in detecting and responding to signals
- Response of a cell to a signal depends on the cell’s particular collection of proteins
- Pathway branching and ‘cross-talk’ further help the cell coordinate incoming signals
Example of negative feedback: homeostasis of blood glucose stimulated due to rising blood glucose (eg after eating a carbohydrate rich meal)?
- Blood glucose level increase
- Beta cells of pancreas stimulated to release insulin into blood
- Insulin causes the liver to take up more glucose and stores it as glycogen, and body cells take up more glucose
- Results in blood glucose level declining to a set point; stimulus for insulin release diminishes.
Example of negative feedback: homeostasis of blood glucose stimulated due to removal of excess glucose from blood resulting in blow blood glucose level (eg after skipping a meal)?
- Decline in blood glucose concentration
- Stimulates alpha cells of pancreas to release glucagon into the blood
- Glucagon causes the liver to break down glycogen and release glucose into the blood
- Resulting in blood glucose level rising to a set point; stimulus for glucagon release diminishes
What did Earl W. Sutherland win the Nobel prize in Physiology Medicine for in 1971?
His discoveries concerning mechanisms of action of hormones
Sutherland suggested that cells receiving signals go through what three processes?
- Reception (Occurs on cell membrane)
- Transduction (Involves several steps)
- Response (Cellular activity due to signal)
What is reception in terms of cell signalling?
Detection by the cell of a signal (molecule) that usually originates from outside the cell
When is a signal detected in cell signalling?
When signalling molecule interacts directly with a receptor on cell surface or inside the cell
(Usually referred to as: receptor binding)
What can ligand binding lead to?
Ligand = signalling molecule (small molecule that binds to a larger one)
Binding can lead to a change in the shape of a protein/aggregation of 2 or more receptors = receptor can interact with other molecules
Hydrophilic messengers versus hydrophobic messengers in cellular signalling?
Hydrophilic = (water soluble and often too large to pass through membrane) detected by membrane bound receptors
Hydrophobic = can move through the lipid environment of the PM and so signal receptors can be located inside the cell
Receptors located in the plasma membrane where mostly water soluble signal molecules bind. What are the three main types of membrane receptors?
G protein-coupled receptors
Receptor tyrosine kinases
Ion channel receptors
What is a GPCR?
- G-Protein-coupled receptors = largest family of cell-surface receptors
- A plasma membrane receptor; spans membrane as 7 a-helices
- Works with the help of a G protein
Where is the NH3+ terminus and COO- terminus located on the GPCR structure?
NH3+ = extracellular flid
COO- = Cytosol
(Signal binding site = extracellular fluid, Segment that interacts with G protein = Cytosol)
G-proteins can bind guanine nucleotides: GTP and GDP. What occurs due to this?
- When GDP is bound to the G-protein - G-protein is inactive – i.e. the switch is OFF
- When GTP is bound, the G-protein is activated - i.e. the switch is ON
What occurs when a signalling molecule binds to extracellular side of GCPR?
Receptor activation
- Activation = change in shape of receptor, cytoplasmic side of GCPR binds to inactive G protein
Interaction of G protein and GCPR - GTP displace GDP, activating the G protein
Activated G proteins dissociated from GCPR and diffuses along membrane and binds to an enzyme, causing what?
Change in shape and activity of enzyme - enzyme activation leads to cellular response
Binding of signalling molecule (ligand) is reversible – binds and dissociates many times - ligand concentration determines how often signalling occurs
What returns G proteins to inactive state and causes the G protein to leave the enzyme? What happens after this?
Changes in enzyme and G protein are only temporary – because the G protein also functions as a GTPase enzyme – i.e. it hydrolyses GTP to GDP
G protein now available for reuse - GTPase function allows pathway to shut down rapidly when signalling molecule isn’t present
What occurs before a signalling molecule binds to extracellular side of GCPR?
G protein - loosely attached to cytoplasmic side of the cell membrane
G protein - acts as a molecular switch ‘either on or off’
On/off state depends upon whether GDP or GTP is bound
Examples of signalling pathways that use G-Protein coupled receptors?
Epinephrine or adrenaline - from adrenal gland - stimulates glycogen breakdown in liver and skeletal muscle during stress
What bacterial diseases use G-protein coupled receptor signalling pathways?
Whooping cough (pertussis), Cholera, Botulism
Bacterial toxins in these cases interfere with normal G protein coupled receptor function
(Also thought that up to 60% of all medicines used today exert their effects by influencing G protein pathways)
What is the receptor tryosine kinases (RTKs)?
- membrane bound receptors; they have intrinsic enzyme activity (within the protein)
- Acts as an a tyrosine kinase; adds phosphate residues onto other proteins
- Can trigger multiple signal transduction pathways at once
- Abnormal functioning is associated with many types of cancers
Receptor tryosine kinase activation step 1?
Before ligand binds receptors exist as monomers
Binding of ligand = 2 receptor monomers to associate with each other.
Association forms a complex (known as a dimer)
Each receptor monomer has:
(Receptor tryosine kinase activation)
(i) extracellular ligand binding site
(ii) membrane spanning region and
(iii) intracellular tail containing multiple tyrosines
Receptor tryosine kinase activation step 2?
Dimerisation activates tryosine kinase region of each monomer
Each tyrosine kinase adds a phosphate from an ATP molecule to a tyrosine on the tail of the other monomer
Receptor tryosine kinase activation step 3?
The receptor is fully activated and recognised by specific relay proteins inside cell
Relay protein binds to specific phosphorylated tyrosine - undergoes structural change - activates the relay protein
Each activated protein triggers a transduction pathway (response)
Where are receptor tryosine kinases usually found working?
Animal Growth Factors
Involved in cell growth and division
Example of abnormal receptor tyrosine kinases (RTK) associated with cancers? What can treat this?
Some breast cancer patients – excessive levels of a RTK called HER2 (Human Epidermal growth factor Receptor 2) – poor prognosis
Herceptin – approved for the treatment of early-stage breast cancer treatment – binds to HER2 on cells and inhibits their growth and division
Herceptin – monoclonal antibody that binds to receptor
What are ligand-gated ion channel receptors?
acts as a gate - structure creates a pore in the plasma membrane that can open or close in response to an extracellular chemical messenger
(When a signal molecule (ligand) binds to receptor, the gate allows specific ions through channel receptor)
Ligand-gated ion channel activation step 1?
Gate is closed until a ligand binds to the receptor
Ligand-gated ion channel activation step 2?
Ligand binds to receptor – gate opens and specific ions can flow through the channel into the cell – this rapidly changes the intracellular concentration of that ion – causes cellular response
Ligand-gated ion channel activation step 3?
When ligand dissociates from the receptor the gate closes and ions can no longer enter the cell
Examples of Ligand-gated ion channels?
Nervous system – e.g. neurotransmitter molecules released at a synapse between 2 nerve cells
Explain how neurotransmitters are an example of ligand-gated ion channel receptors?
Neurotransmitter bind as ligands to ion channels on receiving cell – causes channels to open
Ions flow in (or out) triggering an electrical signal that propagates down the length of the receiving cell
Some gated ion-channels are controlled by electrical signals instead of ligands – voltage-gated ion channels crucial to functioning of the nervous system
Examples of drugs that exert their effects through modulating ion channels? What is their role?
Verapamil - calcium channel blocker - used for treatment of hypertension and other cardiovascular disorders
Lamictal - sodium channel blocker - used for treatment of epilepsy
Lidocaine - sodium channel blocker - local anaesthetic
Glipizide - potassium channel blocker - used in the treatment of diabetes
What are intracellular receptors?
- found in the cytosol or nucleus of target cells
- Small or hydrophobic chemical messengers - readily cross the membrane and activate receptors
- Examples of hydrophobic messengers - steroid and thyroid hormones of animals
- An activated hormone-receptor complex can act as a transcription factor
How do intracellular receptors work? For example with Hormone testosterone? (step 1)
Hormone passes through the plasma membrane – e.g. steroid and thyroid hormones - also nitric oxide (NO)
Testosterone – secreted by cells in testes – the hormone travels through the blood and enters cells all over body
How do intracellular receptors work? For example with Hormone testosterone? (step 2)
ONLY cells with appropriate receptor can respond – but in these cells testosterone activates an intracellular receptor
Hormone-receptor complex enters the nucleus and acts as a transcription factor – mRNA produced – protein synthesis
What is transduction?
Cascades of molecular interactions relay signals from receptors to target molecules in cell
Signal transduction usually involved multiple steps, why?
Multistep pathways can amplify a signal - a few molecules can produce a large cellular response
Multistep pathways provide more opportunities for coordination and regulation of cellular responses
What molecules relay signals from receptor to response?
Proteins
- the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated
- At each step, the signal is transduced into a different form, usually a shape change in a protein
What does protein kinases transfer?
Phosphates from ATP to protein (phosphorylation)
- Phosphorylation most commonly occurs on serine, threonine (or tyrosine) residues
- Phosphorylation normally leads to protein activation
What do protein phosphatases do?
Remove phosphates from proteins (dephosphorylation)
Transduction phosphorylation and dephosphorylation system acts as a…?
Molecular switch - turning activities on and off or up or down, as required.
Transduction pathway activity is regulated by…
Ratio of kinase to phosphatase activity within a cell
What do phosphorylation cascades do?
Activate and inactivate forms of proteins to trigger a cellular response
(provides opportunity for crosstalk)
What are ‘first messengers’
Extracellular signal molecules (ligand) that binds to the receptor
What is a ‘second messenger’?
- small, nonprotein, water-soluble molecules or ions that readily spread throughout a cell by diffusion
- participate in pathways initiated by GPCRs and RTKs
- Cyclic AMP and calcium ions are common second messengers
What converts ATP into cAMP? What can cAMP be broken down by?
Adenylyl cyclase, an enzyme in the plasma membrane, converts ATP to cAMP in response to an extracellular signal
cAMP can be broken down by phosphodiesterase to form AMP, which is inactive
cAMP as a second messenger?
- Many signal molecules trigger formation of cAMP
- Other components of cAMP pathways are G proteins, G protein-coupled receptors, and protein kinases
- usually activates protein kinase A, which phosphorylates various other proteins
- Further regulation of cell metabolism is provided by G-protein systems that inhibit adenylyl cyclase
cAMP as a second messenger in a GPCR signalling pathway
First messaging binds to G protein-coupled receptor, activating the G protein and GTP binds
Activating Adenylyl cyclase, where ATP is transformed into cAMP (second messenger)
cAMP activates Protein kinase A, which will initiate a cellular response
Cholera affecting signal transduction?
disease is initiated by drinking water containing the bacterium Vibrio cholerae
bacteria colonise the small intestine (forming a biofilm) and produce an enzyme that acts as a toxin
Affects a G-protein involved in regulating salt and water excretion
Step 1: cholera affecting signal transduction?
Cholera toxin activates a G protein
This modified G-protein is unable to hydrolyse GTP to GDP - hence is switched ON all the time
Step 2: cholera affecting signal transduction?
This leads to constant activation of adenylate cyclase and continuous production of cAMP
Step 3: cholera affecting signal transduction?
High cAMP levels activate the cystic fibrosis transmembrane conductance regulator (CFTR)
Causes dramatic efflux of Cl-ions and water from infected cells leading to watery diarrhoea
Calcium ions act as a second messenger in pathways. Its important because…?
cells can regulate its concentration
Under normal conditions intracellular calcium concentration is very low
At a concentration that may be 10,000 times lower than in the blood stream (or plant cell wall)
How is cytoplasmic concentration of calcium ions kept low, creating a concentration gradient between extracellular fluid and cytosol?
Why does this make calcium a very good secondary messenger?
Cytoplasmic calcium is actively pumped into the ER and/or mitochondria (or chloroplasts in plants) to keep cytoplasmic concentration of calcium ions low
cells can regulate its concentration –> Pathways leading to the release of calcium involve inositol triphosphate (IP3) and diacylglycerol (DAG) as additional second messengers
Step 1: calcium ions act as a secondary messenger?
Signalling molecule binds to receptor – leads to activation of phospholipase C
Step 2: calcium ions act as a secondary messenger?
Phospholipase C cleaves membrane phospholipid (PIP2) into DAG and IP3
Step 3: calcium ions act as a secondary messenger?
IP3 diffuses through cytosol to an IP3 gated calcium channel in ER membrane – causing it to open
Step 4: calcium ions act as a secondary messenger?
Calcium ions flow out (down concentration gradient) raising cytoplasmic calcium levels
Step 5: calcium ions act as a secondary messenger?
Calcium ions activate next protein in one (or more) signalling pathways
DAG functions as second messenger in other pathways
What is Calmodulin (CaM)?
a very specific calcium modulated protein - contains four Ca2+ binding sites
How does calmodulin become activated?
Calcium binding induces conformational changes – this allows CaM to bind to other proteins - causing activation or inactivation
- What proteins are most often regulated by Calmodulin?
- What else does CaM activate?
protein phosphatases and kinases but CaM also regulates the activity of adenylyl cyclases and phosphodiesterase (cAMP formation/breakdown)
- Plasma membrane Ca2+-ATPase (pumps Ca2+ out of cell) is also activated by CaM (reduces cytoplasmic Ca2+ concentration)
Nuclear and cytoplasmic signalling responses?
a signal transduction pathway leads to the regulation of one or more cellular activities and response occurs in cytoplasm or nucleus
Many signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus
The final activated molecule in the signalling pathway may function as a transcription factor
Simplified signalling pathway that leads to regulation of gene expression in response to a growth hormone?
- Growth factor binds to receptor
- Phosphorylation cascade in cytoplasm
- Activates transcription factor in nucleus, causing mRNA formation in the nucleus
Signalling pathways can affect what?
- activity of enzymes, eg stimulation of glycogen in response to epinephrine
- Activation of expression eg stimulation of transcription factor in response to growth factor
- Overall behaviour of a cell eg changes in cell shape during mating in yeast
Removing Fus3 from a signalling pathway what can occur?
Formin not phosphorylated so not activated,
No initiation of growth of microfilaments forming shmoo projections
No shmoo formed.
What are the 4 aspects of fine-tuning cell response to consider?
Amplifying the signal (and thus the response)
Specificity of the response
Overall efficiency of response, enhanced by scaffolding proteins
Termination of the signal
What are scaffolding proteins in cell signalling?
large relay proteins to which other relay proteins are attached
- can increase signal transduction efficiency by grouping together different proteins involved in the same pathway
How are cell signals terminated?
Inactivation mechanisms are an essential aspect of cell signalling
If the ligand concentration falls then fewer receptors will be bound
The unbound receptors revert to an inactive state
But also remember the example of how Ca2+ and CaM contribute to a negative feedback loop to reduce cytoplasmic Ca2+ levels
Only certain organs, called target organs, respond to the presence of a specific hormone because?
Only those organ cells have the appropriate receptors
A molecule that is not involved in the second messenger system of adrenaline is…?
A. G protein
B. Adenyl cyclase
C. Prostaglandin
D. Protein kinase
E. Cyclin AMP
C. Prostaglandin
An example of a hormone that exerts its action within the target cell only by interacting with cell-surface receptors…?
Insulin
One molecule that can act as both a neurotransmitter and a hormone is…?
Noradrenaline (norepinephrine)
Homeostasis is achieved through the actions of two major regulatory systems, the nervous system and the ________ system.
Endocrine
The action of steroid hormones involve all of the following except:
a. hormone molecules pass through the target cell’s plasma membrane
b. hormones bind to intracellular receptor proteins
c. the hormone-receptor complex binds to specific regions of DNA
d. the hormone-receptor complex activates genes, producing action in the target cells
e. the second messengers activate previously inactive enzymes
e. the second messengers activate previously inactive enzymes
Which one of the following does not represent a category of hormones?
a. secondary messengers
b. polypeptides
c. glycoproteins
d. amines
e. steroid
a. secondary messengers
A ________ is a regulatory chemical that is secreted into the blood by an endocrine gland.
Hormone
Some intercellular regulatory molecules that exert only very local effects are called ________ regulators.
Paracrine
The water-soluble hormones cannot pass through the plasma membrane; they must rely on ______ messengers within their target cells to mediate their action.
second
