Insulin

Question 1

What are the effects of insulin?

Answer 1

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

Question 2

What pathway does insulin signal through and what are the main steps of this pathway?

Answer 2

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

Question 3

What kind of receptors does glucagon act on and where?

Answer 3

Acts on glucagon receptors on the liver
GPCR coupled to Gs subunit
Increases cAMP and PKA

Question 4

What are the effects of glucagon?

Answer 4

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

Question 5

Describe the pattern of insulin release following a meal

Answer 5

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

Question 6

How is insulin release controlled by the nutrient pathway?

Answer 6

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

Question 7

How is insulin release controlled by the neural pathway?

Answer 7

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

Question 8

Describe 3 ‘amplifying pathways’ that regulate insulin secretion independently of glucose

Answer 8

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

Question 9

How are beta cells specialised for insulin secretion?

Answer 9

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

Question 10

What are the 2 types of monogenic diabetes and their causes

Answer 10

MODY - mutation in beta cell transcription factors, or glucokinase
Neonatal diabetes - mutation in ATP-sensitive potassium channel

Question 11

What might you measure when diagnosing diabetes and differentiating between Type 1 and Type 2?

Answer 11

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)

Question 12

What are the current method of glucose monitoring and insulin delivery?

Answer 12

Finger prick testing
Carbohydrate counting
Insulin injections

Question 13

What are the newer methods of glucose monitoring and insulin delivery?

Answer 13

Continuous glucose monitoring
Flash glucose monitoring 
Insulin pump
Insulin analogues 
Smart insulin - microneedle array patches
Inhaled insulin
Artificial pancreas

Question 14

What are the potential methods of beta cell replacement therapy?

Answer 14

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

Question 15

What are the benefits of insulin analogues

Answer 15

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

Question 16

What are the pros and cons of inhaled insulin?

Answer 16

Very rapid absorption

Short duration of action
Needs large device that is not easy to carry around

Question 17

How do microneedle array patches work?

Answer 17

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

Question 18

What are the cons of islet transplants?

Answer 18

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

Question 19

What are the pros of islet transplants?

Answer 19

Less invasive than full pancreas transplant

Improves glucose control, less need for insulin

Question 20

How might the current issues with islet transplants be overcome in the future?

Answer 20

Using animals as sources of islets
Alternative sites of injection other than the liver
Improve culturing of islets before transplant

Question 21

How can embryonic stem cells be used to create new beta cells?

Answer 21

Cells taken from 5-7 day old blastocyst

Cultured and conditions changed to stimulate differentiation into beta cells

Question 22

What are the cons of using embryonic stem cells to create new beta cells?

Answer 22

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

Question 23

How can iPS cells be used to create new beta cells?

Answer 23

Adult cells are de-differentiated into stem cells using transcription factors, and then re-differentiated into beta cells

Question 24

What are the pros (3) and cons (3) of using iPS cells to create new beta cells?

Answer 24

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

Question 25

How can transdifferentiation be used to create new beta cells?

Answer 25

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

Question 26

How can a patients own beta cells be regenerated?

Answer 26

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.