Cell Signalling Flashcards
What are inotropic receptors
Receptor which binds to an agonist which is an ion channel
What is a metabotropic receptor
A receptor that binds a ligand which triggers a cascade of reactions
What are the forces that effect the opening of an ion channel
Chemical gradient and electrical gradient
What is the reverse potential
The point at which the forces are equal = no movement of ions
What is the resting membrane potential
The average of resting potentials of the open ion channels
Which receptors are non specific cation channels
Glutamate and Ach receptors
What are non specific cation channels
Not specific to ions and have a reversal potential close to 0
Where are Ach receptors found
NMJ in muscle cells
Between pre and post synaptic cells in the ANS
What ions are Ach receptors permeable to
Na+, K+, Ca2+
What part of the Ach receptor allows it to bind to Ach
Alpha subunit - each Ach molecule can bind 2 receptors
Name an Ach antagonist and how it works
Alpha-bungarotoxin
Blocks the ability of the nerve to control the muscles
Causes paralysis
What receptors have a pentameric structure
GABA glycine and nicotinic Ach receptors
What is the importance of the amino acids that line the pore of the receptors
They dictate if they let through anions or cations
What is the importance of the lock an key mechanism on receptors
Affect wha can bind/modulate the activity of the receptors
Why are GABA and glycine receptors inhibitory
Ecl is close to Em and below the action potential threshold -> prevents action potentials from happening
Why is subunit composition important
Subtle variations can affect the receptor properties e.g sensitivity to ligands
What do GABA-R subunit composition dictate
Receptor properties, dynamic variations and cell surface distribution
How does the location of the delta subunit in GABA-R relate to its functions
Placed outside of the cell and doesn’t contact much GABA
Monitors the ambient level and persistently open if GABA present
How does the location of the beta subunit in GABA-R relate to its functions
At the synapse and come in contact w lots of GABA
Open at precise moments and quickly close
Describe purinergic receptors
Excitatory receptors w ATP as their ligand
Permeable to cations
Expressed in the brain
What is glutamate
A principal excitatory neurotransmitter in vertebrate nervous systems
Describe ionotropic glutamate receptors
The ion channel is the receptor
Lined w negatively charged amino acid subunits
Allows cation to pass through to generate an excitatory subunit
What are the 3 main classes of glutathione receptors that are defined by man made agonists
AMPA, NMDA, kainate receptors
Which agonists activate all 3 classes of glutamate receptors
Glutamate and kainate
Where are AMPARs and NMDARs usually co-localised
At synapses where they mediate fast chemical synaptic transmission
Why are AMPARs and NMDARs needed at the presynaptic terminal
To act as auto receptors -> glutamate binds on the presynaptic site
Describe the structure of iGluRs
Have 4 subunits and 3 transmembrane domains (TM1, TM3, TM4) with a re-entrant loop (TM2)
What is the importance of the extracellular domain in iGluRs
Create the glutamate binding site
What is the importance of the transmembrane loops in iGluRs
Create the shape of the pore
Important for various intracellular processes
Where does glutamate bind on iGluRs
Between the S and S2 extracellular domains
Each subunit can bind 1 molecule
What subunits make up AMPARs
GluA1 - GluA4
Variants of the same protein that bind together to form the receptor
What mutations can the AMPARs subunits undergo
Alternative splicing and RNA editing
What channels can the AMPARs form
Homomeric; 1 type of subunit
Heteromeric; mix of subunits
Why is the GluA2 subunit important in AMPARs
It determines the current/voltage curve
Dictates the reversal potential and affects Ca2+ permeability
What are the AMPARs splice variants
2 exon; flip and flop which can affect receptor kinetics
What determines the rate of AMPARs desensitisation to glutamate
The subunit composition and flip/flop variant
Which subunit is affected by flip and how
The GluA4 subunit
Flip variant lengthens time the receptor is open
Describe how the AMPAR becomes desensitised to glutamate
When glutamate is maintained at receptor it doesn’t maintain current
The ion channel close even though glutamate is bound
Which subunits of AMPAR are permeable to Ca2+
GluA1 and GluA3
Why are many AMPARs impermeable to Ca2+
They contain a GluA2 channel
How can GluA2s permeability to Ca2+ be altered
Glutamine at the pore is flipped to arginine
What dictates the functionality of NMDARs
Mg2+
Drawn in by the negative field which depolarises cell
Negative field now has less influence so Mg2+ forced out
Leaves the pore open
Compare the I vs V relationship for AMPARs and NMDARs
AMPAR; linear relationship
NMDAR; non linear relationship
How is the NMDAR conditional
Requires the presence of glutamate and for the cell to depolarise
What is the purpose of phosphoinositide signalling
To keep [cystolic Ca2+] low to prevent Ca2+ and excess phosphates bonding to form a precipitate
How does the Ca-ATPase pump work
Pumps Ca2+ out of the cell using energy from ATP -> ADP hydrolysis against Ca2+ conc gradient
What is the Ca-ATPase in the plasma membrane called
PMCA
What is the Ca-ATPase in the ER called
SERCA
What does the SERCA pump do
Pump Ca2+ from the cytosol into the ER or the SR in a muscle cell
How does the Na/Ca exchanger work
Removes Ca2+ from the cell by exchanging them for Na+
Uses inward Na+ gradient generated by Na/K pump
Why is the mitochondria important in maintaining a low cystolic Ca2+ conc
Take up Ca2+ when the SERCA and PMCA become saturated
Describe the function of calmodulin
Sensor protein which undergoes conformational change when bound to Ca2+
Trigger downstream physiological responses
What is the function of buffer proteins
Bind to Ca2+ and act as aa sponge to bring down cytosolic [Ca2+]
Why does the cell need a range of mechanisms to bring down cystolic [Ca2+]
So the cells can remove the ions over a wide range of [Ca2+]
Where are IP3Rs and RyRs located
On the membrane of ER/SR
What is the secondary messenger for IP3Rs
IP3 and Ca2+
What is the secondary messenger for RyRs
Ca2+
What are the 3 types of Ca2+ signals
Elementary events
Global Ca2+ wave intracellular
Global wave Ca2+ intercellular
Describe an elementary Ca2+ event
Very small and doesn’t take up the entire cell
Ca2+ rise is highly located in the cell
Describe a global Ca2+ wave intracellular
Ca2+ waves happens in part of the cell and makes it way a cross the whole cell
Describe a global Ca2+ wave intercellular
The Ca2+ wave doesn’t stop at the boundary of the cell it propagates across the mono layer of cells
Describe how a signal binding to a GCPR causes Ca2+ release from the ER/SR
Signal binds GCPR -> activates phospholipase C-beta
Clips PIP2 molecule -> DAG and IP3
IP3 released into cytosol and diffuses to IP3R on ER/SR
Opens channel to release Ca2+ from ER/SR
What happens to DAG when it’s produced from PIP2
Stuck in the plasma membrane and tries to find PKC its target protein
Why does DAG remain in the plasma membrane
Due to its hydrophobic fatty acid chains
How does a Ca2+ signal present itself
Series of spikes that will persist as long as a the hormone is present
What effect will doubling the [hormone] on the Ca2+ spikes
Amplitude remains constant
Frequency will increase
Describe CICR
Ca2+ released from the ER stimulates further Ca2+ release from neighbouring IP3Rs
Feeds forward to release more Ca2+
What are the 2 components to a Ca2+ signal
Temporal (spike) and spatial (wave)
Describe how a Ca2+ spike and wave are linked
When spike begins = initiation of Ca2+ propagation
Midway through spike = mid propagation halfway through the cell
Peak = whole cell is full of Ca2+
What buffers/prevents the diffusion of Ca2+ through the cell
Buffered by binding to buffer proteins = no wave/spike
What needs to happen to generate a Ca2+ wave/spike
CICR needs to overcome the buffer proteins momentarily to generate wave/spikes
What needs to take place for the falling phase of the Ca2+ spike
IP3Rs stop releasing Ca2+ to allow the off mechanisms to remove Ca2+
What happens in between Ca2+ spikes
Cell at rest and off mechanisms relax which allow the cell to produce another Ca2+ spike
What is the suggested stimulus for RyRs
CADPR
Why is IP3 required for CICR to be triggered
To sensitise the receptor and Ca2+ for CICR
How does RyRs differ from IP3Rs
RyRs don’t need cADPR to sensitise the receptor in order to behave as a CICR channel
IP3Rs need IP3 to be able to behave as a CICR channel
Why do RyRs and IP3Rs require ATP
As aa safety mechanism
How does ATP work as aa safety mechanism for RyRs and IP3Rs
If cell has plenty energy then there will be ATP present
If low ATP - site wont be occupied on receptor so no movement of Ca2+
= no Ca2+ signalling
What is a Ca2+ puff
When Ca2+ is released from a cluster of 10 IP3Rs
What is a Ca2+ spark
When Ca2+ is released from a cluster of 10 RyRs
What is an abortive Ca2+ wave
When the wave is borderline before intermediate/high level of hormones
Wave only propagates across part of the cell not the full wave
How are puffs and sparks forerunners for a Ca2+
Sensitise the IP3R so that if the hormone stimulus increases, the channels can produce a CICR
What is a Ca2+ blip
A fundamental event released as a result from only 1 IP3R
What is a quark
A fundamental event released as a result from only 1 RyR
How can a Ca2+ dependent response be triggered i.e wave not necessary
Ca2+ spark in muscle -> Sr releases Ca2+ near membrane
Spark close to the Ca2+ activated K+ channel
Activates the channel = cell hyperpolarisation = muscle relaxation
How does a global Ca2+ effect differ to aa spark in muscle cells
Global = contraction
Spark = relaxation
How can a Ca2+ wave propagate though cells
IP3 and Ca2+ are needed to diffuse through gap junctions
How do the cilia of different cells coordinate a response
Through Ca2+ intracellular wave events
Describe a cell specific calcium signalsome
Defines the precise isoforms of each component that is any given cell
What are the 2 types of remodelling signalsomes undergo
Phenotype and genotypes remodelling
What is phenotypic remodelling of a signalsome
when the component become phosphorylated which can change its activity
What is genotypic remodelling of a signalsome
Somatic mutations in single cells alter activity of a component and are passes on from one generation to the next via germline mutations
Describe how phenotypic remodelling allows the heart to have stronger/larger contractions
CAMP dependent reversible phosphorylation of key Ca2+ signalling components
= enables more Ca2+ to enter the cell and more pumped out to generate larger Ca2+ signals
What drives liver regeneration
Down regulation of key Ca2+ signalling components
How does Alzheimer’s disease affect memory
Extracellular plaque deposits of beta amyloid peptide which aggravates outside of nervous cells to disrupt synaptic transmission
How does APP and beta amyloid affect the cytosolic Ca2+ levels
Up regulation of Ca2+ signalling
Beta amyloid oligomers increase Ca2+ entry via the NMDAR
APP increase Ca2+ release from stores
What happens to APP once it has been metabolised
Makes its way to the nucleus to act as a TF and increase the txn of RyRs and decrease txn of calbindin
How does APP result in amyloid dependent Ca2+ signalling
Increase Ca2+ leak through RyR into cytosol
Decrease in Ca2+ buffering
= [Ca2+] in nervous cells
How are memories formed
Triggered by a strong Ca2+ signal through NMDAR through the LTP
Initially enter temporary memory store but are consolidated into the permanent memory store at night
How does Ca2+ erase the temporary memory store
Broader raise of Ca2+ but at a lower signal triggers LTD
Erases temporal memory store so it is ready to receive new temporary memories
How does Alzheimer’s affect memory
Increased level of resting Ca2+ so there is still LTP during the day but there is persistent activation of LTD
= erasing of temporary memory store before it can be consolidated to permanent store
How can vitamin D potentially reverse Ca2+ dependent neuro-degeneration
Increases the expression of plasma membrane ATPase and Na/Ca+ exchanger
Increases [calbindin]
= decrease in resting cytosolic [Ca2+]
How does positive feedback make it hard to perceive the primary cause of Alzheimer’s
Amyloid stimulates an increase in Ca2+ which can stimulate APP which also increases Ca2+
What is Brody myopathy
Skeletal muscle genetic disorder characterised by stiffness and cramp
What causes Brody myopathy
Mutation in SERCA1pump therefore can’t pump out all the Ca2 back into SR
Impedes relaxation
What are the 2 types of adenylyl cyclase
Plasma membrane form
Soluble form
How is the membrane bound from of adenylyl cyclase activated
By the alpha s subunit on GPCR which causes a conformational change
Allows the catalytic subunits to come together
What is the reaction to form cAMP
ATP -> cAMP and pyrophosphate via adenylyl cyclase
How can forskolin activate all isoforms
Bypasses receptor - GCPR complex and binds directly to adenylyl cyclase
What is the role of PDEs
To breakdown cAMP and some can breakdown cGMP
Why are PDEs important
Can affect the duration of cAMP rise and how the cAMP level can rise
Alerts the spatial localisation of the cAMP signal
What are the 3 off mechanisms for cAMP signalling
PDEs
Inhibit cAMP production
CAMP removal
How can cAMP production be inhibited
Some GCPR agonist activate the Gai to reduce AC activity by binding to AC
How can cAMP be removed from the cell
MRP transporters that pump cAMP out of the cell
What affect does cAMP removal have on the signal
Affects the duration and amplitude of signal
Can impact on spatial aspects
What suggests that cAMP signalling is compartmentalised to micro domains and are agonist specific
Different agonists increase cAMP levels but produce different response in same cell
Some agonists produce cAMP dependent responses but do not change global cAMP levels
How can cAMP signalling be compartmentalised
AKAPs bind to PKA and have a target domain which allows them to be brung closer to their substrates
Help assemble signalsomes
What are the 2 forms of PKA
Type 1 and type 2
Describe type 1 PKA
Inactive PKAA binds to cAMP binding domains which releases the catalytic domain to phosphorylate other molecules
Describe type 2 PKA
Bind to AKAPS via docking domain but catalytic domain isn’t released
AKAPS bring PKA closer to the substrate
Where are beta adrenergic receptors located in a muscle cell and why
Localised near the T tubule where all the excitatory contraction machinery is located
Why do prostanoid receptors excitation not lead to a contraction
Localised in different regions and phosphorylate metabolism enzymes not contraction machinery
What leads to PKA dependent phosphorylation of key Ca2+ components in a muscle cell
Beta adrenergic stimulation -> increased elevations in cAMP
Ha
