Insulin Flashcards
What are the effects of insulin?
Insertion of GLUT4 transporters into fat and muscle cells
Increases the activity of glucokinase and glycogen synthase to promote glycolysis and glycogenesis
Inhibits glycogenolysis and gluconeogenesis
Inhibits lipolysis
What pathway does insulin signal through and what are the main steps of this pathway?
PI3K/Akt pathway
Insulin binds insulin receptor (tyrosine kinase)
Dimerisation and transautophosphorylation
Recruitment and phosphorylation of IRS-1
IRS-1 activates PI3K
PI3K phosphorylates PIP2 into PIP3
PIP3 activates protein kinase B
What kind of receptors does glucagon act on and where?
Acts on glucagon receptors on the liver
GPCR coupled to Gs subunit
Increases cAMP and PKA
What are the effects of glucagon?
Raises blood glucose levels through…
Increases the activity of glycogen phosphorylase to promote glycogenolysis
Promotion of gluconeogenesis
Promotion of lipolysis
Inhibition of glycogen synthase to inhibit glycogenesis
Inhibition of glycolysis
Describe the pattern of insulin release following a meal
Initial transient burst of insulin release due to exocytosis of granules directly adjacent to beta cell membrane
Continuous lower release of stored and newly synthesised insulin
How is insulin release controlled by the nutrient pathway?
Glucose uptake into beta cells through GLUT2
Metabolised by glucokinase during glycolysis
Production of ATP
Closure of ATP sensitive potassium channels
Cell depolarises
Calcium influx
Vesicle fusion with membrane
How is insulin release controlled by the neural pathway?
PSNS secretes Ach, Ach binds a GPCR that activates phospholipase C, activation of protein kinase C promotes vesicle fusion
SNS secretes NA, NA binds adrenergic receptor that inhibits formation of cAMP, less protein kinase A leads to less vesicle fusion
Describe 3 ‘amplifying pathways’ that regulate insulin secretion independently of glucose
Fatty acids metabolised by krebs cycle to produce ATP, close channels…
Also bind to cell surface receptor GPR40 which increases protein kinase C and A
GLP-1 hormone secreted by gut when eating, increases cAMP and PKA
Amino acids metabolised by krebs cycle, direct depolarisation of cell membrane
How are beta cells specialised for insulin secretion?
Glucokinase sets insulin secretion threshold as it has low affinity for glucose, only acts when levels are high
Glucokinase not inhibited by it’s product at high concentrations
GLUT2 transporter also has low glucose affinity
Pancreatic islets are highly vascularised for accurate blood glucose detection
Beta cell numbers are tightly controlled
Beta cells are highly differentiated and have many unique transcription factors
What are the 2 types of monogenic diabetes and their causes
MODY - mutation in beta cell transcription factors, or glucokinase
Neonatal diabetes - mutation in ATP-sensitive potassium channel
What might you measure when diagnosing diabetes and differentiating between Type 1 and Type 2?
Blood glucose levels
HbA1c (glycated haemoglobin)
Anti-GAD antibodies or islet cell antibodies
Serum C-peptide (preproinsulin cleaved into insulin and c-peptide during production in beta cells)
What are the current method of glucose monitoring and insulin delivery?
Finger prick testing
Carbohydrate counting
Insulin injections
What are the newer methods of glucose monitoring and insulin delivery?
Continuous glucose monitoring Flash glucose monitoring Insulin pump Insulin analogues Smart insulin - microneedle array patches Inhaled insulin Artificial pancreas
What are the potential methods of beta cell replacement therapy?
Whole pancreas/Islet transplants
Creation of new beta cells for transplant (using embryonic stem cells, induced pluripotent stem cells, or transdifferentiation)
Regeneration of the patient’s beta cells
What are the benefits of insulin analogues
Duration of action can be increased by
- Raising the isoelectric point to reduce solubility at physiological pH
- Attachment of a fatty acid tail so it binds to albumin in the plasma
Or onset can be made more rapid by sequence changes that prevent dimer formation
What are the pros and cons of inhaled insulin?
Very rapid absorption
Short duration of action
Needs large device that is not easy to carry around
How do microneedle array patches work?
Patch contains insulin nanoparticles and glucose oxidase surrounded by a polymer. As glucose levels rise, glucose oxidase activity increases and causes the environment to become hypoxic, so the polymer disassembles and releases the insulin
What are the cons of islet transplants?
Immunosuppression required for rest of life which has side effects
High levels of cell death due to injecting into the liver, multiple transplants required
Insulin injections might still be needed
Limited supply of donors
Infection
What are the pros of islet transplants?
Less invasive than full pancreas transplant
Improves glucose control, less need for insulin
How might the current issues with islet transplants be overcome in the future?
Using animals as sources of islets
Alternative sites of injection other than the liver
Improve culturing of islets before transplant
How can embryonic stem cells be used to create new beta cells?
Cells taken from 5-7 day old blastocyst
Cultured and conditions changed to stimulate differentiation into beta cells
What are the cons of using embryonic stem cells to create new beta cells?
Ethical issues
Continued autoimmune destruction of the new beta cells
Genetic modification carries cancer risk
The process is difficult
High levels of heterogeneity in the cells produced - full maturation can require transplant into an animal model first
How can iPS cells be used to create new beta cells?
Adult cells are de-differentiated into stem cells using transcription factors, and then re-differentiated into beta cells
What are the pros (3) and cons (3) of using iPS cells to create new beta cells?
Less ethical issues than using embryonic stem cells
Potentially unlimited supply
Cells can also be used to study beta cell defects
Technically difficult process
Cancer risk
Costly in terms of time and money
How can transdifferentiation be used to create new beta cells?
Adult liver cells, such as alpha cells, can be transdifferentiated into beta cells by using a virus to deliver transcription factors to them such as PDX-1, Ngn3 and Mafa
How can a patients own beta cells be regenerated?
Patients that still have some detectable levels of C-peptide have some remaining beta cells. Application of IGF-1, hepatocyte growth factor, adenosine agonists or GABA may increase their proliferation. Also the fasting-mimicking diet may promote proliferation.
