Lecture 9: Receptors linked to enzymes Flashcards
Tuesday 28th January 2025
Preamble, from last lecture
A startling clinical trial showed that severe reduction in calorific intake combined with vigorous exercise resulted in:
Weight loss (eat less, do more) – no great surprise
Reduction in symptoms of Type II diabetes
A restoration of insulin sensitivity in Type II diabetics
Since blood glucose levels were lowered in these patients, and subsequently insulin sensitivity was restored, this suggests that high blood glucose levels desensitise insulin signalling.
This has generated the notion of intermittent fasting diets:
the 5:2 diet – eat normally for 5 days, then reduce eating (a lot) for 2 days, etc
the every-other-day diet (eat, fast, eat, fast …)
They seem to work
Is it true that people tend to have difficulty maintaining dietary approaches for long periods?
Yes, it’s true
What is the calculation for body mass index?
Estimated by Body Mass Index (BMI) = weight (kg)/height x height (m2)
What does a BMI over 30 indicate?
Obesity
What does a BMI between 25 and 30 indicate?
Overweight
What does a BMI below 25 indicate?
Normal
Who first described BMI and when?
Described in 1835 by Lambert Adolphe Jacques Quetelet.
Is BMI an accurate measurement of health and why?
- No, BMI is not an accurate measurement of health.
- It is a bizarre crude measure that was used when there were no calculators – so had to be simple to be usable.
- It gives poor estimates for tall people (they appear to be too fat), for short people (they appear to be too thin) and for those with an athletic build (who appear to be too fat).
Does the World Health Organisation publish worldwide prevalence of obesity?
Yes, but stopped publishing average BMI per country in 2016
The average person in the US is…
The AVERAGE person in the US is now ‘overweight’ (but the females better watch out).
The average person in the UK is probably…
The AVERAGE person in the UK is probably ‘overweight’ - the rate of increase in BMI may be slowing.
The average male in China is…
The AVERAGE male in China may have been overweight in 2021. The AVERAGE Chinese female is probably of normal weight.
What is obesity?
Obesity is the result of intake of excess calories, more than are consumed by the body’s activities
What are the mechanisms for dealing with excess calorific intake?
a) Conversion of excess fuel to fat that is stored in adipose tissue.
b) Increased locomotor activity: excess fuel is burnt by extra exercise
c) Thermogenesis: conversion of excess fuel to heat
In increased locomotor activity an efficient way to reduce calorific content?
No
In mammals, what maintains adipose tissue at a suitable level?
In mammals there is a complex set of hormonal and neuronal signals that maintains adipose tissue at a suitable level.
Describe the Lipostat theory (1953)
The Lipostat Theory, proposed in 1953, suggests that the body regulates weight through a “set point” mechanism controlled by adipose (fat) tissue. According to the theory:
- Eating behavior is inhibited when body weight exceeds a certain threshold (the set point).
- Energy consumption increases when body weight is above this set point.
- Increased fat storage (adipose tissue) leads to feedback signals that suppress appetite (signals like “I’m full”).
- When adipose tissue decreases, the reverse happens—hunger signals increase, and fat oxidation (burning) decreases to conserve energy.
- Essentially, this theory explains how the body maintains a stable weight by balancing food intake and energy expenditure based on fat stores.
What is evidence for the lipostat model of body mass regulation?
- Leptin is released into the bloodstream by adipose tissue.
- Leptin binds to leptin receptors in the hypothalamus, and changes feeding behaviour i.e a stop eating message
What type of signalling is leptin signalling?
Endocrine signalling
In which animal was leptin first discovered in?
In mice
Is it true that the number and size of adipocytes is increased in Lepob/Lepob mice compared to in LepOB/LepOB mice?
Yes
Is it true that even though they are obese, Lepob/Lepob mice display the physiology and behaviour of starvation?
Yes
What features of starvation showcase?
raised cortisol levels (stress response)
hypothermic (have difficulty staying warm)
low fertility
impaired wound healing
unrestrained appetite
is it true that the starvation features of Lepob/Lepob mice overlap with the features of type II diabetes?
Yes. For example, metabolic disturbances: hyperglycaemic and insulin resistant
One single leptin injection in obese mice?
Lack of leptin is corrected and reduces the phenotype if an obese mouse.
Long-term correction of obesity and diabetes in genetically obese mice by a single intramuscular injection of recombinant adeno-associated virus encoding mouse leptin.
Long-term correction of obesity and diabetes in genetically obese mice by a single intramuscular injection of recombinant adeno-associated virus encoding mouse leptin.
Is it true that humans can be injected with recombinant leptin to decrease obesity?
Yes
Mice with diabetes have a mutated form of the wild type LeprDB/LeprDB gene and instead have Leprdb/ Leprdb. What are the diabetic symptoms that they have?
- obese
- insulin resistant
- elevated plasma insulin levels
- elevated blood glucose levels
- polyuric
- severe depletion of the insulin-producing β-cells of the pancreatic islets
- dead by 10 months of age.
- poor wound healing
- do not respond to leptin injection (because they lack the leptin receptor)
So do the mice that lack the leptin receptor have a type II diabetes phenotype?
Yes
So what does leptin do?
- It is released by adipose tissue and it carries a message that our fat reserves are sufficient.
- It binds receptors in specific neurons (the anorexigenic or appetite-reducing neurons) in the hypothalamus, and stimulates a signalling cascade that results in release of a hormone (α-MSH, alpha-melanocyte stimulating hormone) that modulates nervous transmission.
- The effects are:
- suppression of appetite (eat less)
- stimulation of the sympathetic nervous system (do more)
- increased blood pressure
- increased heart rate
- increased thermogenesis
How does leptin bind to its reecptor?
- Leptin binds and stimulates the dimerisation of leptin Receptor.
- That dimerisation creares binding sites for protein called JAK.
- JAK can now find the leptin receptor and bind to it.
- JAK is a soluble kinase that will phoshphorylates its targets.
- JAK bnds to the leptin receptor tails to activate JAK.
- JAK phiosphorlyats the receptor that it’s bound to.
- These phosphates are binding sites for molecules called STATS, and also the fat stats. (wehich are involved in regulating body weight).
- STATS are signal transducers and activators of transcription.
- JAK phosphorylates the STATS.
- When there’s lots pf phosphates, they’re often used as a dimmer switch.
- Once, they’re phosphorylated, the STATS dimerise, exposing their nuclear localisation signals. (NLS).
- The dimerised STATS are now able to pass through nuclear pore and into the nucleus.
- As they’re transcription factors, they bind to the promoters of a specific set of genes and turn those genes on.
- In this case, the gene makes the precursor for α-MSH, whoch is then matured and sends a signal to the next neurone to send a stop eating, do more signal to the brain.
So given all the evidence, is type II diabetes limited to just the insulin pathway?
No. there’s cross talk between 3 receptors: the EGF receptor, the insulin receptor, and the leptin receptor.
Draw out cross-talk diagram
Draw out cross-talk diagram
What is Erythropoietin ?
Erythropoietin (EPO) is a hormone cytokine that controls the development of erythrocytes (red blood cells) from precursor cells in the bone marrow.
What is unusual about Erythropoietin?
Erythropoietin is unusual among the hematopoietic growth factors because it is produced primarily in the kidneys rather than the bone marrow.
How do the kidneys regulate EPO function?
The kidney monitors hematocrit (the percentage of blood occupied by red blood cells) and maintains it at around 45%.
If oxygen levels drop (e.g., due to high altitude or blood loss), the kidney secretes more EPO to stimulate red blood cell production.
What are the NICE (National Institutes for Health and Care Excellence) guidelines on using EPO?
- Erythropoietin analogues with iron injections are recommended as a possible treatment for anaemia caused by cancer treatment only in:
- women receiving platinum-based chemotherapy for cancer of the ovaries who have a blood haemoglobin level of 8 g/100 ml or lower
- people who have very severe anaemia and cannot receive blood transfusions.
What is the molecular weight of human EPO?
43kDa
What percentage of human EPO weight is from glycosylation?
Approx 40%
How can synthetic EPO be prepared?
Synthetic EPO can be prepared from tissue culture – but even then, subtle differences in glycosylation can be detected.
When is synthetic EPO first though to have been used in professional cycling?
In 1990. It has continued to be used until recently and has been used in other sport as well, such as boxing and distance running.
How does EPO signal?
- EPO binds to the EPO receptor and this brings the 2 cytosolic domains closer together.
- Signalling now occurs via 2 pathways.
- One of the pathways is via a JAK-STAT pathway, using STAT5.
- The interaction of the cytosolic domains allows the recruitment of a JAK.
- The JAK phosphorylates the EPO receptor.
- This phosphorylation allows for recruitment of the STAT.
- The JAK phposphorylates the STAT.
- The stat dimersies and its nuclear localisation signals become exposed, and STAT then goes into the nucelus.
- In the nucleus, the STAT turns on the development of red blood cells from precursors in the bone marrow.
- Though this JAK can also phosphorylate its partner JAK.
- This phosphorylated JAK can be recognised by the adaptors Grb2 and Sos
- This will then recruit Ras and the Ras-dependant growth pathway.
- Both of these pathways allow for the correction of low red blood cell levels. (but not high red blood cell levels).
EPO -> Development
EGF -> Mitogenesis
Insulin -> Altered metabolism
Leptin -> ‘Stop eating, do more’
All controlled by cell signalling
EPO -> Development
EGF -> Mitogenesis
Insulin -> Altered metabolism
Leptin -> ‘Stop eating, do more’
All controlled by cell signalling
Hence, signalling can co-ordinate development, growth and changes in metabolism with behaviour.
Hence, signalling can co-ordinate development, growth and changes in metabolism with behaviour.
