cell signalling Flashcards
Why do cells communicate?
Essential for survival
Blood glucose
Infection
Wounding
Time to contract
Time to divide
Time to die
And many, many more
Cell Signalling
Specific cells have specific receptors for specific signals. When a receptor detects a signal it triggers a cellular response.
Different cell types express different receptors and are capable of responding to specific signals. Only certain cells with certain receptors respond to certain signals.
when there is no signal
Usually tightly controlled.
Too much signal or response without a signal e.g. cancer
No longer responding to or detecting a signal e.g. diabetes or immuno-deficiency
examples of a small molecule, peptides and steroids
Different kinds of signalling
- Direct – connexin proteins form pores between adjacent cells. Cells share cytosol & cytoplasmic molecule and ion concentrations. Also cell contact with the extracellular matrix via integrins.
- Paracrine – short range signals. Cells communicate with adjacent cells. e.g. cytokines released from immune cells or blood vessel endothelium
VEGF = vascular endothelial growth factor. Oxygen sensing – hypoxia leads to the expression of VEGF which leads to angiogenesis (formation of new blood vessels). Nobel Prize for Physiology or Medicine 2019 for the scientists who worked out the oxygen sensing pathway!
3.Endocrine – hormone secreted by endocrine gland into the blood. Binds to receptors on a target cell.
- Synaptic. Neuronal signalling.
- Autocrine is a cell responding to a signal released by itself – e.g. a cytokine released from an immune cell, binding to cytokine receptors on the same cell’s membrane.
The four receptor classes
Receptors are proteins.
RESPONSES
Change protein production -Turn genes on or off
Change protein activity - Turn enzymes on or off, reorganise the cytoskeleton
steroid receptor e.g
Steroids eg. testosterone are all derived from cholesterol and so is lipid soluble and can diffuse across the cell membrane.
Where would you expect to find testosterone receptors?
Immunofluorescence microscopy image – different proteins labelled with fluorescent labels.
Steroid receptors are transcription activators /transcription factors. They cut out the middle man.
what are steroids and their main classes, e.g andgland secreted from
synthesised from cholesterol
Steroids e.g. testosterone are all derived from cholesterol and so are lipid soluble and can diffuse across the cell membrane.
Aldosterone – long term regulation of blood pressure
Cortisol – stress hormone
Progestins – synthetic hormones that mimic the endogenous progesterone hormone
how steriod hormones work
how STEROID RECEPTORS ACTIVATE GENES DIRECTLY
- So cells control availability of genes by chromatin / histones but also by specific transcription activators (or transcription factors) binding to specific DNA elements.
- The specific gene switched on can be downstream many nucleotides away!
- The same enhancer regions can be present upstream of many different genes so steroids can lead to transcription of many different genes.
GPCRs
regulate all number of signals e.g. the migration of leukocytes to infection, fight or flight (flight in this case – adrenaline), and even photons of light
Second messengers
Kinase activation
Links to ion channels
Genome is about 20,000 genes. 800-1000 receptors.
1 in 20 human genes code for a GPCR
800 receptors!
40% of pharmaceuticals
All sorts of signals mediated by GPCRs
Chemokines, olfaction, many hormones, light!
Adrenaline…..
what are GPCRs made of
Proteins (they do everything remember?) – span membranes – due to the way they are produced on the Rough ER they can do this! Here is a GPCR doing just that.
They have hydrophobic bits which are happy in the lipid bilayer of the membrane.
But the important thing to get here is that they provide a link between the outside of the cell and the inside – the signal can pass across!
ADRENALINE - contrasting effects:
Metabotropic Receptors - G-Protein-Coupled Receptors (GPCRs):
Structure:
G-Proteins:
Activation Mechanism:
Types of Alpha Subunits:
- Structure: GPCRs are also known as serpentine receptors because they span the cell membrane (plasmalemma) seven times.
- G-Proteins:
These are GTP-binding proteins and have a heterotrimeric structure (composed of three subunits: alpha (α), beta (β), and gamma (γ)).
The alpha (α) subunit has GTPase activity, meaning it can break down GTP to GDP. The GTP-bound state is the “ON” state of the G-protein. - Activation Mechanism:
When a ligand binds to the GPCR, GDP is exchanged for GTP on the alpha (α) subunit of the G-protein. - Types of Alpha Subunits:
Gαs (stimulatory): Activates adenylate cyclase (AC), leading to the production of cyclic AMP (cAMP).
Gαi (inhibitory): Inhibits adenylate cyclase, reducing cAMP production.
Gαq: Activates phospholipase C (PLC), which generates second messengers.
Gαo: Can directly stimulate or inhibit ligand-gated ion channels, with effects often mediated by βγ subunits; important in brain signaling.
ADRENALINE
one hormone - several receptors!
See pages 60-64 of Costanzo’s Physiology https://librarysearch.exeter.ac.uk/permalink/44UOEX_INST/5mg45k/alma991011231819707446
Costanzo’s Physiology is a fantastic book and will be really useful to you for your IHP learning.
β1 receptors in the heart
β2 in the lung
alpha and beta receptors
Coupled to heterotrimeric G proteins: Alpha, beta, gamma subunits
G proteins act like a simple binary switch. GDP bound – OFF. GTP bound – ON
Bind the ligand (hormone e.g. adrenaline) – change receptor shape. Changes the G protein structure = leads to release of GDP and binding of GTP.
G proteins are enzymes – GTPases. They will turn themselves off by hydrolysing GTP to GDP
Gαs, Gαq, DAG and Ca2+
Gαs
stimulatory. Activates the enzyme adenylate cyclase which converts ATP into cAMP. cAMP activates the kinase enzyme protein kinase A
Gαq
”quirky one”. Activates a lipase (phospholipase C), which cuts phospholipids in the cell membrane into IP3 and DAG. IP3 binds to the IP3R (IP3 receptor) on the ER/SR. These IP3Rs are ligand gated calcium channels found on the ER – leads to release of calcium from the intracellular stores.
DAG and Ca2+
activate the kinase enzyme protein kinase C (PKC)
The second messenger Ca2+ released from the SR, downstream of α1-adrenergic receptor signalling initiates smooth muscle contraction.
abbreviations
cAMP
CaM – calmodulin
DAG diacylglycerol
IP3Inositol trisphosphate
PKA protein kinase A (an effector)
PKC protein kinase C (an effector)
PLC phospholipase C
beta cells
what do the secondary messgers made by alpha and beta do?
G proteins are enzymes – GTPases. They will turn themselves off by hydrolysing GTP to GDP
TURNING THE SIGNAL OFF
TURNING THE SIGNAL OFF - PDE 3 inhibitors used to treat heart failure
the different sub-types
Quick notes for memory aids:
S = stimulatory. Activates AC (adenylate cyclase), increases intracellular cAMP which activates PKA (protein kinase A). PKA is also knows as cyclic-AMP dependent protein kinase.
Note cAMP is quickly broken down by cAMP phosphodiesterases to ensure the signal is not always ON.
I = inhibitory
Q = the “quirky one” – doesn’t signal through AC or cAMP. Activates PLC - activates PKC (protein kinase C)
IP3 = inositol trisphosphate
IP3R = ligand gated ion channel. The ligand is IP3 . The ion that flows through the channel when it opens after ligand binding is Ca2+
ER/SR = endoplasmic/sarcoplasmic reticulum
DAG =diacylglycerol
KINASES
ADD PHOSPHATE TO THINGS
Glycogen phosphorylase breaks up the glycogen polysaccharide (energy storage in the liver and skeletal muscles) into glucose
CARDIAC MUSCLE
L-type calcium channel is a voltage gated channel found on the cell membrane (sarcolemma) of muscle cells. PKA adds phosphate groups to the cytoplasmic region of this channel and increases the likelihood of the channel opening. Increases heart rate.
BETA BLOCKERS
Resting Membrane Potential
The electrochemical gradient needs the energy of ATP hydrolysis which drives the Na+/K+ pump and plasma membrane calcium ATPases (PMCAs) and SERCA pumps on the ER/SR organelle membrane.
https://cvphysiology.com/arrhythmias/a007 – this website and its sister website cv pharmacology are fantastic resources for understanding cardiovascular physiology
NEUROMUSCULAR JUNCTION
KINASES ADD PHOSPHATE TO THINGS
Receptor tyrosine kinase or RTK
Example = EGF =epidermal growth factor, binds to the EGF receptor. Receptor dimerises. Transautophosphorylation (kinase domain phosphorylates tyrosine residues on the other monomer). Phosphotyrosines on the cytoplasmic domain attract adapter proteins which lead the activation of Ras.
Ras is a small GTPase. It breaks down GTP to GDP. When activated it will initiate the Mitogen activate protein kinase cascade (MAPK)
Ras is a G protein. ON – GTP bound, OFF GDP
TIME
SPACE
Alpha adrenergic receptor mechanism in smooth muscle e.g. the gut…
SMOOTH MUSCLE
Calmodulin is a calcium sensing protein. When bound to Ca2+ it changes shape and can interact with many downstream proteins. One of these, MLCK or myosin light chain kinase, is triggered to stimulate smooth muscle contraction when bound by calmodulin-Ca2+
Beta receptors in SMOOTH MUSCLE
In smooth muscle, PKA can inhibit the activity of myosin light chain kinase which reduces smooth muscle contraction.
Receptor types in the pituitary hypothalamus axis
So to highlight the pituitary – hypothalamus – gonad axis you may have covered in the last case we have GPCRs and steroid receptors involved.
More details of the EGF receptor/Ras pathway
How Ras is turned on and what it does! Using the EGF receptor as an example.
EGF binds to the EGF receptor (EGFR). One EGF binds two receptors and so brings them in close proximity – dimerisation.
The EGFRs are, of course, proteins; each one a single polypeptide chain that spans the membrane. The protein folds in such a way that the extracellular region can bind to EGF and the intracellular region has the enzymatic activity of a tyrosine kinase.
As you will remember, a tyrosine kinase will add phosphates to specific tyrosine residues in a target protein(s). In this case the target is the other EGFR protein that is dimerised by the binding of EGF.
The resulting phospho-tyrosine ‘tags’ act as binding sites for numerous other proteins in the cell. I have drawn just the most famous one here –Grb2.
Grb2 has a binding site that only recognises phosphorylated growth factor receptors – it is therefore localised to the receptor following EGF binding. This is an example of phosphorylation affecting the sub-cellular localisation of target proteins.
Why is this important?
Ras is usually found associated with the inside of the cell membrane.
Grb2 binds to another protein called Sos
When Grb2 binds to an activated receptor it delivers Sos to the membrane
Sos is a GTP exchange factor for Ras – ie it facilitates the binding of GTP of Ras to turn it on.
Active Ras can now bind to and activate other signalling molecules. The first of these is called Raf. It is a serine/threonine kinase and it phosphorylates related kinases (MAP kinases) which phosphorylate other kinases and so on. This is called a MAP kinase cascade and amplifies the signal.
One of the targets of this cascade is a transcription factor. When this becomes phosphorylated it can move to the nucleus and stimulate the transcription of several genes that code for the proteins required for cell division.
testosterone
The FSH receptor
Showing the amino acid sequence. The N-terminus (start) of the protein is the M at the top.
Photoreceptor cells in the retina detect light using rhodopsin coupled to GaT
Cell membranes are impermeable to ions (eg. Na+) which must pass through protein channels (green) to move between the inside and outside of the cell
In the absence of light cyclicGMP predominates in the cytoplasm which keeps one such Na+ channel open
