cell communication

Question 1

what are the four kinds of cell signalling?

Answer 1

Contact dependent - one signalling cell - membrane-bound signal - one target cell

Synaptic - can be one-to-one cell communication (like contact-dependent), tho over longer distances

Paracrine - local mediator sent out - affecting the closely surrounding cells

Endocrine - wider reaching, hormone released into blood stream travelling all over tho only affecting target cells with the corresponding receptors
(Some signals can travel further - pheromones from queen bees send out signals that control tens of thousands of worker bees)

Question 2

explain how the notch pathway is an example of contact-dependent signalling

Answer 2

used in development

Seen in drosophila eyes in order to correctly space R8 receptors, there should be 8 in each segment

Notch is the receptor, with its ligand called delta

Neighbouring cells decide which is going to become an R8 receptor and which aren’t using notch signalling

The one that is going to be an R8 cell uses contact dependent signalling and expresses the delta ligand, preventing neighbouring cells (expressing the notch receptor) from developing into R8 receptors to ensure the desired even distribution. known as lateral inhibition

Question 3

two common ways cells respond to signalling?

Answer 3

often some change in gene expression

or maybe a change in cytoskeleton e.g. to move (macrophages)

Question 4

explain how hedgehog signalling and DPP signalling are examples of paracrine signalling

Question 5

does a signal always result in the same response?

Answer 5

NO
a response can be dependant on the dose, there is often a max response level

can be concentration dependent

different depending on the receptor

Question 6

how are morphogens an example of quantitative responses to a signal?

Answer 6

in high concentrations, morphogens result in a certain cell type forming, and different cell type at lower concentrations

Bicoid for example, is found in drosophila embryos in a gradient to ensure the front of the fly develops as the front

Question 7

rank three common kinds of response to a signal in order of which is quickest (include an example)

Answer 7

Alter structure of protein like an ion channel - causing it to open (super quick)

Post translational modification - e.g. phosphorylation (pretty quick)

Change protein levels by affecting gene expression (slow) - for example yeast cells being put in galactose rather than glucose and having to produce different enzymes as a result

Question 8

polycythaemia - how is this an example of signalling gone wrong?

Answer 8

it is a myeloproliferative neoplasm (blood disorder with overproduction of one of the blood cells)

a single A.a change in the JAK2 protein - which normally regulates red blood cell production - results in permanently ‘on’ JAK2 and therefore way to many red blood cells

Question 9

how is chronic myeloid leukaemia an example of signalling gone wrong?

Treatment?

Answer 9

gene mutation (translocation) BCR gene fuses with Abl (from Chr 9 and Chr 22).

Abl encodes a tyrosine kinase, and here its promoter region is affected to be constantly on (treated by imatinib an Abl tyrosine kinase inhibitor)

Question 10

from fucking flies again, give an example of positive feedback

Answer 10

notch pathway???

Question 11

fast responses to a signal require rapid turnover of the effector. how do we make this efficient?

Answer 11

instead of:
making a load of protein when you want the signal, then destroying all that protein to turn off the signal, then making it again when you need the signal etc…

the body uses processes like phosphorylation to “turn on” proteins and later turn them off without destroying and remaking them, similarly G coupled protein receptors exchanging GDP for GTP when activated, and hydrolysing back to GDP when no longer in use

Question 12

using monomeric GTPases (not GPCRs) is efficient. how do they work?

Answer 12

This is a protein that requires a GTP to be bound to it in order to be active, and shows how proteins just need to be modified in some way to turn off and on, it is used in many processes, including translation

A GEF - GTP exchange factor - is needed to help our monomeric GTPase get rid of that GDP and replace it with another GTP, to become ACTIVE

Once it’s done its thing it needs to go back to inactive

GAP, a GTPase activating protein, is needed to help our monomeric GTPase hydrolyse its GTP back to GDP (its intrinsic GTPase activity is low). Now the GTP is once again inactive

Question 13

give an example of how we target these processes with drugs when they go wrong

Answer 13

CML - tyrosine kinase constantly on

use imatinib, a tyrosine kinase inhibitor

polycaethemia - ruxolitinib inhibits the tyrosine kinase JAK2

Question 14

martin’s medicine - what is bisoprolol?

Answer 14

selective B1 adrenergic receptor inhibitor, used to treat certain heart conditions

fun fact - it can cross the BB barrier and cause nightmares!

Question 15

how does the body simplify signalling (3 ways)?

Answer 15

reduce complexity, for example neurons using the same receptor and signal on different cells for different effects

You only need one type of cell to produce the one hormone, and the target cells can express the same receptor, but each target cell type having different downstream effects - this saves effort

hormones work at very low concentrations

Question 16

what areas of the brain are key in hormone production?

Answer 16

the hypothalamus and the pituitary gland

Question 17

why is cholesterol used as a precursor for making hormones?

Answer 17

Hydrophobic lipid tail, hydrophilic -OH group, so can cross membranes, B/B barrier

Many hormones and also vitamins (vitamin D, cortisol, oestrogen, testosterone etc…) need the ability to cross these kinds of barriers

Question 18

what are the two categories of steroid hormones?

Answer 18

corticosteroids - like cortisol, made in adrenal cortex -
1. Glucocorticoids,
2. mineralocorticoids,

sex steroids - made in gonads/placenta
oestrogens, progestogens, testosterone (androgens)

Question 19

what domains are there in a nuclear receptor?

Answer 19

N and C terminal domains
Ligand binding domain
Hinge region
DNA binding domain

Question 20

describe the structure and function of the DNA binding domain of a nuclear hormone receptor?

Answer 20

zinc fingers containing a zinc that interacts with 4 cysteine residues, these are the DNA binding domains, in order to reach into double helix major grooves of DNA

Question 21

nuclear hormone receptors -what are they like when inactive and how do they become active?

Answer 21

They are essentially hormone dependent nuclear transcription factors
Receptor is either inactive, when the DNA domain at the N terminus end is not working due to bound inhibitory proteins

Ligand binds, causing conformational change that means the zinc finger loses the inhibitory proteins and can then ‘lock in’ the hormone ligand
This ‘locking’ mechanism contributes to high specificity
The hinge region can now bend causing another conformational change, coactivator proteins can join and the DNA binding site is in position to bind to the major groove of DNA etc…

Question 22

once active, what do nuclear hormone receptors do?

Answer 22

Always first cause production of primary response proteins that will then modulate other genes, either negatively or positively, to form secondary response proteins that will have the additional, desired physiological effects - it is essentially just a cascade

Question 23

cortisol - what are its therapeutic uses?

when is it released by the body and what does it effect?

Answer 23

as a glucocorticoid it
Can be used as an immunosuppressant and anti inflammatory agent

Widely different effects on different organ systems so not to be used lightly

Released in response to stress and low blood sugar

Affects metabolism, immune system, electrolyte balance and memory

Question 24

what is the hypothalamic-pituitary-adrenal axis?

Answer 24

Hyp - corticotropin releasing hormone (CRH)

Pituitary then releases adrenocorticotropic hormone (ACTH)

Triggers adrenal gland to make cortisol

Question 25

what happens when you have too little cortisol (disease)?

include the distinction between primary and secondary forms of the disease

Answer 25

Addison’s disease, primary version is due to autoimmune response damaging adrenal glands, secondary version is due to a lack of ACTH (give someone ACTH to differentiate between the two)

If your body experiences stress (an accident, an operation, an infection etc…) and it cannot respond, addisonian crisis occurs, super low blood sugar and some other severe symptoms, body shuts down

Question 26

what is Cushing’s syndrome?

Answer 26

too much cortisol

often caused by tumour of pituitary gland = too much ACTH = too much cortisol (also known as dexamethasone)
Also caused by long term steroid abuse, can cause raised blood pressure and weight gain and puffy face, you stop responding to own cortisol so issues occur if athletes who take cortisol decide to stop

Question 27

why is hypoglycaemia so dangerous?

Answer 27

(too much insulin)

brain can only metabolise glucose - so super low blood sugar is very dangerous

Question 28

type I diabetes - what is it? what causes it?

what are the physiological effects and symptoms?

Answer 28

high blood sugar over prolonged periods (hyperglycaemia)

due to destruction of beta cells in islets of Langerhans often following viral infection that causes an autoimmune response = no insulin being produced

glucose in blood not taken up = effecting osmotic potential
glucagon depletion - Without the inhibitory effects of insulin, the alpha cells in the pancreas release more glucagon. This excess glucagon, exacerbates hyperglycemia
can cause polyuria and hypertension

risk increases as BMI increases

Question 29

type II diabetes - what is it? what is it caused by?

how is it treated?

is it curable?

what are the symptoms?

Answer 29

low insulin production but mainly peripheral insulin resistance (desensitised receptors) which can be linked to obesity

treatment -
Some treated with insulin/insulin stimulating drugs as its levels are lowered, but also drugs to improve efficacy of the resistant receptors known as biguanide drugs

not curable but can be prediabetic and take measures to reduce obesity and ‘turn it around’

symptoms -
Increased thirst, hunger, urination, fatigue and blurred vision

Question 30

explain the clinical trial showing rigorous exercise and diet control could potentially ‘reverse’ diabetes

Answer 30

Type II diabetes patients

One group had typical care

Other group had rigorous exercise and diet to manage weight, no medicines

Results showed the second group lost more weight than the other group and had more people who reversed their diabetes (40% after 12 months, a bit less after 24 months)

Mostly white people tho (some evidence suggest diabetes in south asian people is less linked to BMI)

Question 31

what did flies show when insulin signalling was studied?

Answer 31

insulin signalling is conserved in animals

flies with restricted insulin signalling actually lived longer, possibly due to caloric constriction - for them, eating less extended life

Question 32

how does metformin work when used to treat diabetes?

Answer 32

Metformin inhibits mitochondrial complex I, preventing the production of mitochondrial ATP leading to increased cytoplasmic ADP : ATP and AMP : ATP ratios

These changes activate AMP-activated protein kinase (AMPK), an enzyme that plays an important role in the regulation of glucose metabolism

Question 33

explain the structure of the insulin receptor

Answer 33

The receptor is a heterotetramer - it has four subunits, two alpha ones and two beta ones
They are linked by disulphide bonds
The alpha ones are extracellular and have a ligand binding site
The beta ones are transmembrane subunits, with their ends in the cytoplasm exhibiting tyrosine kinase activity

Question 34

how does the insulin receptor work?

Answer 34

Receptor activation initiates a cascade of phosphorylation events that leads to the activation of enzymes which control many aspects of metabolism and growth

1. Ligand binds causing a conformational change, dimerisation
2. Kinase domains ‘trans-phosphorylate’
3. At the same time, an associated docking protein known as ‘ISR 1’ (insulin receptor substrate 1) also gets phosphorylated at a tyrosine residue
4. Once phosphorylated, IRS binds to lipid kinase PI3K, which synthesises PIP3 at the plasma membrane
5. PIP3 recruits PDK (another kinase) which phosphorylates AKT
6. AKT then phosphorylates lots of things at both Ser and Thr residues, including transcription factors and glycogen synthase kinase

Long story short, when activated it causes a cascade of many phosphorylations

Question 35

describe the structure of receptor tyrosine kinases

Answer 35

Hydrophilic extracellular domain to interact with the ligand
Most will have ‘conserved’ elements that are immunoglobulin like

Transmembrane domain - 20-25 Aa of alpha helical structure, hydrophobic, stabilised by lipid bilayer fatty acid chains
Has to be trafficked to the membrane - this is done by signal-peptides

Intracellular domain - hydrophilic, has tyrosine kinase activity to interact with downstream machinery

Question 36

how do receptor tyrosine kinases typically work?

Answer 36

Dimerisation of single-pass transmembrane domains (pulls extracellular domains of two receptors together so they can do something e.g. phosphorylate one another)

Tyrosine kinase receptors have enzymatic activity themselves, often phosphorylating each other upon dimerisation (working in ‘trans’), resulting in a phosphorylation cascade of some kind

Question 37

take a breath -

explain the steps in the Ras/Raf pathway when the receptor is activated by PDGF (platelet-derived growth factor)

Answer 37

ligand binding = dimerisation = trans phosphorylation

from here it goes Ras/Raf/MEK/ERK

The sites on the receptor that get phosphorylated are used as docking domains for SH2 domains (a domain present in many proteins for the purpose of binding to RTKs)
In this case the SH2 domain of the ‘adaptor protein’ Grb2
Grb2 then ‘grabs’ Sos - a GTP to GDP exchange factor - GEF
Sos can then swap GDP to GTP in the membrane associated Ras molecule - activating it

This membrane -tethered Ras has GTPase activity
This means it naturally would switch itself off by getting the GTP back to GDP
This will only be possible when Sos is no longer around
The active Ras-GTP binds to Raf
This results in loss of Raf’s inhibitory domain and Raf is now active
Raf phosphorylates itself (this is now Ser/Thr phosphorylation)
Raf phosphorylates MEK,
which does the same to ERK - this goes into the nucleus to activate transcription factors

Question 38

give three examples of how the Ras/Raf pathway is regulated (focusing on Raf)

Answer 38

Raf is regulated at many points in this pathway

1. Raf is regulated by its own N-terminal region
2. Also regulated by the 14-3-3 protein
3. Erk - the final kinase - also is involved in negative regulation and can phosphorylate Sos to prevent it being ‘grabbed’ by Grb2

Question 39

what causes cutaneous melanomas and how are they treated?

is it effective?

Answer 39

In the Ras/Raf pathway, issues can lead to cutaneous melanomas
Inhibitors of B-RAF’s kinase activity have been developed to treat malignant melanoma
Very effective at first but even if tiny percentage of mutated B-RAF kinases aren’t inhibited those cells survive and grow
So if extracellular signalling is fine, it still means intracellular signalling is over-working
B-RAF and MEK (further downstream) are both targeted with inhibitors as its very unlikely that there cells with resistant forms of both these proteins

Question 40

B-RAF inhibitors often result in relapse - how is this improved?

Answer 40

B-RAF and MEK (further downstream) are both targeted with inhibitors as its very unlikely that there are cells with resistant forms of both these proteins

Question 41

in the JAK/STAT pathway, how does the receptor work/what does it respond to etc…?

Answer 41

So there’s a cytokine receptor on a T cell - which is a single pass transmembrane receptor
These receptors bind to cytokines or MHC molecules - molecules on cells that have bound to and are presenting antigens to the T cells

The receptor is associated with this is JAK, which is a tyrosine kinase (Don’t have the tyrosine kinase built in, use JAK, an associated tyrosine kinase)

Binding of ligand = dimerization and transphosphorylation

Question 42

(JAK/STAT): when the cytokine receptor has been activated, what is the downstream pathway?

Answer 42

The phosphorylation allows SH2 domains of STAT to bind
STAT is phosphorylated, goes to the nucleus, binds DNA and results in transcription

transcription of genes involved in cell proliferation for example

Question 43

how has the body kept JAK/STAT signalling less complex?

Answer 43

there’s only 4 JAKs (kinases) and 7 STATs

the pathway can be activated by different ligands, and have different downstream effects (so the pathway/molecules in the pathway are very versatile)

Question 44

how is JAK regulated?

how ‘much’ does JAK work when healthy vs in disease?

Answer 44

In JAK - Pseudokinase domain negatively regulates the kinase site

JAK 2 works at 1% max capacity if uninhibited
For disease to be reached it only needs to be 8%

Question 45

what disease is associated with a mutation in JAK?

Answer 45

myeloproliferative neoplasms -
can cause production of too many
1. red blood cells - polycythaemia vera
2. platelets - essential thrombocythemia

or too few red blood cells due to scaring -
3. myelofibrosis