Cell communication Flashcards
What is contact dependent signalling?
membrane bound signal molecule attaches onto target cell
What is paracrine signalling?
signalling cell produces local mediators that affect surrounding target cells
What is endocrine signalling?
endocrine cell produces hormone that is widely acting and circulate through the body to impact target organs
What is a morphogen gradient?
responses occur at specific thresholds instead of qualitatively, there are uncommitted or committed cells
Positive feedback
short stimulus causes fast response becauase the products positively regulate each other so the system maxes out and is sustained beyond stimulus
Negative feedback
stimulus causes response which maxes out but then goes away when the stimulus does as one product is negatively regulating the other
How do kinases work?
control signalling by phsophorylation and work by inhibiting ATP binding pockets so it can’t bind, preventing a response
What are the three essential functions of ion channels?
transport of ions across membrane
regulation of membrane potentials in nerve and muscle cells
influx of Ca2+ into cytoplasm for contraction/secretion
Common ion channel structure
transmembrane domains usually made of 2 or more alpha helices
2-6 subunits that surround the pore
What is a simple K+ ion channel made up of?
TM helicase structures that form a p-loop that is highly selective as the cytoplasmic side TMs are more tightly packed- selectivity filter
S1-4 TMs, S5+6 form p-loop
What are factors that can control a simple K+ channel?
membrane potential
mechanical stress
ligands
What are the two main functions of voltage gated ion channels?
to create action potentials in excitable cells
transport Ca2+ into the cytoplasm as a 2nd messenger
What is different about voltage gated channels struture compared to simple ones?
additional helices S1 and S4 form a seperate voltage sensing domain lateral to subunits
large polypeptides that extend into cytoplasm
plugging mechanism
What are transient receptor potential channels?
share common structural features with voltage gated but evolved to sense chemicals and stimuli
Ligand gated channels
similar to voltage gated but are controlled by the binding of a ligand, either intracellular or extracellular
3 out of 4 subunits must be bound to induce response
What types of channels have cytoplasmic anchors?
voltage gated
TRP
ligand gated
Cys-loop type receptors
pentametric
nAChrs
cys loop
5 subunits with 4 TMs each
chronic exposure leads to receptor upregulation
in muscle
What do mutations in nAChrs cause?
M2 region of human a4 neuronal subunit mutated
autosomal dominant nocturnal frontal lobe epilepsy due to receptors having delayed repsonse to NTs, causing a lag and no recycling of NTs which increases their concentration
Types of glutamate receptors
Kainate
NMDA
AMPA
What are NMDA receptors important for?
controlling synaptic plasticity
mediating learning and memory
Glutamate receptors
trimeric, similar structure to ACh but pore is inverted and binding site is a cleft that closes when occupied
vital to every aspect of brain function
P2X receptors
trimeric and are ATP gated ion channels
3 subunits and 2 TM helices with a large extracellular domain
3 ATP molecules needed to open channel
General structural features of GPCRs
7 transmembrane domains which are all alpha helices
TM 3- centrally located next to binding pocket
extracellular N terminus for ligand binding
intracellular C terminus for G protein binding
G proteins
guanine nucleotide binding proteins
belong to the GTPase family
heterotrimeric- alpha, beta and gamma domains
How do G proteins act as molecular switches?
hydrolyse GTP to GDP
GTP- on
GDP- off
How are GPCR classes distinguished?
structural features of the extracellular domains defining the ligand binding site
linked directly to the diversity of stimuli GPCRs can detect
Protease activated receptors
proteases activate receptors in platelets by cleavage of the N terminal, which acts as a tethered ligand
part of the receptor itself acts as an agonist
platelet activation by- thrombin, ADP + basal lamina
How are GPCRs turned on?
inactive state is when GDP is bound to alpha subunit
ligand binds causing conformational change that activates G protein
GDP released and alpha subunit seperates from beta/gamma to bind GTP
now is active and binds to target protein in membrane to elicit response
How do G proteins act as timers?
the duration of signalling activated by G proteins is regulated by the rate of GDP hydrolysis by the alpha subunit
RGS proteins stimulate GTPase activity in alpha subunit
How many families of G proteins exist?
6
GPCRs compared to ion channels
they are slow to open or close
but stay open/closed for longer
How can active G proteins regulate the activities of enzymes that control 2nd messengers?
small molecules carrying signals to hydrophobic lipids confined to the membrane
small soluble molecules that diffuse throught the cytoplasm like cAMP and cGMP
calcium
How are mutations in GPCRs linked to cancer?
90% of uveal melanomas have mutations in Gq alpha subunits
leads to blocking of GTP hydrolysis so subunits are always active- permanent signal transduction of growth pathways
Adenylate cyclase
has 10 isoforms and is a membrane anchored enzyme
activated by Gs alpha subunit
inhibited by Gi alpha subunit
Gs mechanism
ligand bind to receptor activating g protein
alpha subunit moves and binds to adenylate cyclase in the membrane
activated adenylate cyclase catalyses formation of cAMP from ATP
cAMP activates PKA which then phosphorylates and activated protein to initiate response in cell
How is Gs signalling switched off?
agonist dissociating from receptor
GTPase activity on Gs
cAMP broken down by phosphodiesterase
dephosphorylation of enzymes
cGMP second messenger system
similar to cAMP except enzyme is guanylate cyclase which is receptor bound or free in cytoplasm
converts GTP to 3’-5’ cyclic guanosine monophosphate
What is the local concentration of 2nd messengers determined by?
rate of production
rate of diffusion from site of production
rate of removal
What molecules regulate concentrations of cAMP?
production is regulated by adenylyl cyclase
removal is by phosphodiesterase
What 2 types of messengers can be generated by phospholipase C?
IP3- water soluble and diffuse through the cytoplasm
DAG- hydrophobic molecules that remain in the membrane
Where does specificity in signalling come from?
selective expression and cellular localisation of signalling molecules
Protein Kinase Cs
Ser/Thr kinases that are activated by DAG and Ca2+
PMA phorbol ester
an analogue of DAG used in research to activate PKCs
DAG binding mechanism
causes dissociation of intramolecular psuedosubstrate domain from the active site
once activated PKCs provide either positive or negative feedback in pathway
Phosphorylation of phospholipase C
provides negative feedback for GPCR signalling and makes it transient
What cellular functions does calcium regulate?
synaptic transmission
hormone secretion/synthesis
fertilisation
muscle contraction
cytokinesis
Cytosolic Ca2+ in resting cells
kept low at 100nM by ATP driven Ca2+ pumps
this is important for its job as a 2nd messenger
How is Ca2+ concentration regulated?
influx regulated by channels in the extracellular membrane and ligand gated ion channels on ER
How are ER calcium and store levels refilled/maintained?
store operated channels made up of ORAI and gated by STIM
What does overstimulation of GPCRs lead to?
tachyphylaxis (LSD or salbutamol)
disease such as uncontrolled growth in cancer
How is overstimulation of GPCRs prevented?
GRK (receptor kinases)
B-arrestin
stops G protein from binding and internalises receptor so its degraded or recycled
What channels are an exception to the standard 2-6 subunits that surround the pore usually?
chloride, water and ammonia channels
exist in the middle of a single subunit
What are ion channels classified into subgroups based on?
gating mechanism (voltage or ligand)
ion seletivity of the pore, defined by the physical size of the filter and the amino acids that line it
Alpha helices
right hand helix conformation
Beta sheets
beta strands connected laterally by at least 2 or 3 backbone hydrogen bonds forming a sheet
Subunit
single protein that joins with others to form protein complex
Transmembrane domain
protein that spans the width of the membrane form the extracellular to intracellular sides usually with a helical shape
P loop or pore
pocket where an ion will bind
What types of channels have plugging mechanisms?
voltage gated
TRP
Na+/K+ selective channels
control membrane excitability and depolarise cells
Ca2+ channels
directly regulate the activity of calcium sensitive proteins
Cl- selective channel
control membrane excitability by reducing resistance and hyperpolarising cells to reduce AP firing
a4b2
neuronal nAChRs abundantly expressed in the cortex and hippocampus
have a high affinity to agonists nicotine and varenicline
What is linked to tobacco dependance but better cessation outcomes?
CHRNA4 (a4)
CHRNA6 (a6)
AMPA
mediate fast excitatory synaptic transmission in the CNS
NMDA
involved in learning and memory and is slower than other isoforms
Kainate
similar to AMPA but lesser role at synapses
linked to schizophrenia, depression and huntingtons
RNA splicing in AMPA
each subunit exists as 2 splicing isoforms flip + flop
What does alternative splicing of 2 exons in the primary transcript lead to? (AMPA)
2 protein isoforms with different domains in the extracellular loop that have different kinetic properties
What are the different kinetic properties of flip and flop?
flop has a faster desensitisation rate and reduced current responses to glutamate than flip
What is the effect of a lack of GluA2 RNA editing?
mutant mice lacming enzyme responsible for RNA editing prone to seizures and early death
GluA2 Q/R site
located in the M2 of subunit inside the channel pore
Glu codon to a Arg codon
Downregulation of GluA2 Q/R editing in motor neurons of ALS patients leads to…
increase in Ca2+ permeable AMPA receptors
causes damage due to glutamate excitotoxicity
What is decreased ADAR activity linked to?
increased malignancy in glioblastoma
increase in Ca2+ means Akt pathway promoting proliferation/tumourigenesis is increased
reversed when GluA2 Q/R was edited- potential target
Diseases/physiological conditions linked to PX2/trimeric receptors
P2X2 hearing loss
P2X4 pain
P2X7 inflammation + neurodegenerative disease
Diseases/physiological conditions linked to glutamate/tetrameric receptors
excess NMDA in stroke = neuron death
Disease linked to cys loop receptors
epilepsy
Lipid kinases
add phosphate groups to lipids
What do loss of function mutations in Orai1 cause?
have an important role in activation of t lymphocytes so causes severe combined immunodeficiency
