Modeling Insulin and Diabetes

By Alexander Tsao & Fiona Wang

Alexander Tsao - Insulin, Insulin Processing, Insulin & C-peptide's Effect on ATP Production, Vasodilation, How Diabetes Affects These Processes

Fiona Wang - Introduction to Diabetes, Circulatory System, Glucose-Insulin Regulatory System, How Diabetes Affects These Processes

All images & animations made by us.

Project Summary

Listed as the seventh leading cause of death in the United States, diabetes affects more than 35 million Americans, and 463 million people worldwide. Due to its rapidly increasing prevalence, we decided to explore diabetes and its effects on the body. The models and animations throughout the project illustrate a variety of biological processes, including insulin processing, insulin's role in the production of ATP, vasodilation and the circulatory system, and the glucose-insulin regulatory system. These processes are viewed through the lens of a patient with diabetes, showing how they are affected and inhibited by the disease.

Introduction to Diabetes
Insulin
Insulin Processing
Insulin & C-peptide's Effect on ATP Production
Vasodilation

6. Circulatory System

7. Glucose-Insulin Regulatory System

8. How Diabetes Affects These Processes

9. Works Cited

What is Diabetes?

Diabetes Mellitus, or Diabetes, is a chronic (long-lasting) disease where the body's ability to produce or respond to the hormone insulin is impared, resulting in the abnormal metabolism of carbohydrates and elevated levels of glucose in the blood and urine.

Diabetes can lead to numerous health problems including: loss of vision, kidney & heart failure, and damage to the nervous and circulatory systems. There is currently no cure for diabetes, but there are several treatments or lifestyle changes that can be implemented to help patients mitigate the disorder.

There are several distinct types of the disease, each presenting its own causes & risk factors:

Type 1 Diabetes is an autoimmune disorder where the beta cells in the pancreas are being attacks by the body's own cells, leading to the body producing insufficient amount of insulin. Type 1 diabetes is often inherited, and is most commonly diagnosed in children & young adults who were born with it.
- - Symptoms: increased thirst & urination, constant hunger, weight loss, blurred vision, fatigue
  - Risk Factors: genetic, environmental, auto-immune factors
  - Treatment: daily insulin injections & regular check up of blood sugar levels
Type 2 Diabetes is a diet-related disorder where the pancreas is able to produce insulin, but the body is unable to use this insulin to control blood sugar levels. This is known as insulin resistance. Type 2 diabetes is most frequently related to obesity and a sedentary lifestyle, and is most often diagnosed in adults.
- - Most common form of diabetes (present in >85% of patients)
  - Symptoms: fatigue, frequent urination (especially at night), unusual thirst, weight loss, blurred vision, frequent infections
  - Risk factors: obesity, physical inactivity, high/low birth weights, metabolic syndrome
  - Treatment: diet, exercise, weight loss, mediciation
  - Indicated by blood sugar levels of >125 mg/dL in a fasting plasma glucose test, and >6.4% in a hemoglobin A1c test
Prediabetes occurs when blood sugar levels are elevated, but not high enough to be diagnosed as type 2 diabetes. Symptoms are usually not visible during the prediabetes stage.
- - In a fasting plasma glucose test, prediabetes is indicated with blood sugar levels of 100-125 mg/dL (normal levels: <100 mg/dL)
    - - Fasting plasma glucose test - A test where a blood sample is taken after the patient has refrained from eating for 8 hours to analyze its blood sugar levels
  - In a Hemoglobin A1c test, prediabetes is indicated with blood sugar levels of 5.7-6.4% (normal levels: <5.7%)
    - - Hemoglobin A1c test - A blood test that shows one's average blood sugar levels for the past 2-3 months
Gestational Diabetes forms during pregnancy
- - Often goes away after pregnancy

How prevalent is diabetes?

According to the National Diabetes Statistics Report by the Center for Disease Control and Prevention (CDC), 34.2 million Americans - just over 1 in 10 - had diabetes in 2018. Out of those 34.2 million Americans:

nearly 1.6 million had type 1 diabetes, that of which included around 187,000 children and adolescents
26.8 million were diagnosed, and 7.3 million were undiagnosed
26.8% were seniors, aged 65 or older
about 210,000 Americans under age 20 are estimated to have diagnosed diabetes

*Additionally, these statistics don't account for the 1.5 million Americans being diagnosed with diabetes each year, and 88 milllion Americans aged 18 and older that have prediabetes.

Number of Diabetes Cases in Countries Around the World (2000)

What is Insulin?

In the pancreas, insulin is produced from beta cells in response to the blood sugar released by the consumption of food
- - Beta cells - cells found in pancreatic islets that produce/release insulin and amylin (part of glycemic regulation), responding to spikes in blood glucose levels
  - Increases in blood glucose are detected by the pancreas
Insulin helps to transfer energy from glucose into cells
- - Stimulates glucose uptake by tissues (i.e. liver, fat, muscle)
Without insulin, glucose stays in the blood
- - Hyperglycemia - when the body doesn’t use enough or produce enough insulin
Insulin is also used as a growth factor - a group of proteins that stimulates cell growth
- - Controls transcription
  - Stimulates translocation in proteins
  - Stimulates cell growth and replication
  - Stimulates DNA synthesis

For insulin to be processed, it must go through proteolytic post-translational modification:

What is Proteolytic Post-Translational Modification?

Some enzymes are activated by post-translational modifications
- - Post-translational modifications - alterations that happen to the enzyme after it has been translated
They are synthesized as inactive precursors called proenzymes
- - Proenzymes - these contain an extra fragment that inactivates the enzyme
A proteolytic enzyme cleaves the inactivating fragment from the active enzyme
- - Proteolytic enzymes - enzymes that break apart protein molecules into shorter peptides and further into amino acids
Cleaving the proenzyme exposes the active site on the active enzyme
- - Active site - the region of an enzyme where molecules can bind, undergoing chemical reactions

The active enzyme is what is used to facilitate chemical reactions in the cell.

The inactivating fragment is not used by the cell after it has been cleaved. It's purpose was to stabilize and inactivate the active enzyme.

However, in insulin processing, the inactivating fragment does serve an important purpose in other biological functions.

Why does this process occur?

When a protein is initially produced in an inactive state, it allows a cell to store large amounts of the protein without harming the cell
When the protein is needed in the cell, a large amount of it can be activated quickly

As insulin is processed in a pancreatic cell, it travels from the endoplasmic reticulum to the golgi system to secretory vesicles:

The process begins in the cell's cytoplasm:

Insulin’s precursor is produced from the pancreas as a 110 amino acid molecule called Pre-Pro-Insulin
- Pre-pro-insulin has an N-terminal and C-terminal
  - On a protein, the amino acid residue with an amine group on the alpha carbon is the N-terminus
  - On a protein, the amino acid residue with a carboxylic acid group on the alpha carbon is the C-terminus
On the N-terminus is the Signal Peptide
- Signal peptide - the side of the chain that directs pre-pro-insulin to its secretion pathway

Pre-pro-insulin is tranlated at the surface of the endoplasmic reticulum (ER) and pulled through the ER membrane
This cleaves the signal peptide from pre-pro-insulin

Inside the Lumen of the Endoplasmic Reticulum:

Pro-insulin is formed in the lumen of the ER
Pro-insulin’s N-terminal and C-terminal are connected by disulfide bonds
- Disulfide bonds - bonds that stabilize protein structure
Proinsulin takes on a looped shape

Pro-insulin transits to the Golgi System, then to Secretory Vesicles:

Inside the secretory vesicles, insulin undergoes proteolytic cleavage
Two endopeptidases cleave proinsulin at the c-terminal side of amino acids Arg31/Arg32 and Lys64/Arg65
- Endopeptidase - enzyme that breaks down peptide bonds
Active Insulin is formed
- Active Insulin - the N-terminal and C-terminal regions of proinsulin connected by disulfide bonds
- Made up of two chains: A & B

Proteolysis also produces a C-peptide
- C-peptide - a single polypeptide chain made up of the region of proinsulin that connects the N-terminal and C-terminal

Pancreatic cells secrete both Insulin and C- peptide:

Insulin and c-peptide are transported out of the cell by a secretory vesicle to be used in the body
- Both will be essential to numerous biological processes, such as ATP production, the circulatory system, and the glucose-insulin regulatory system

For a long time, it was thought that the c-peptide was an inactive byproduct of insulin production
More recently, it was discovered that c-peptide had many vital roles and is a hormonally active peptide

From studying red blood cells from organisms with type 1 diabetes and type 2 diabetes, it was found that insulin and c-peptides have an effect on ATP production:

ATP (Adenosine Triphosphate) is an energy carrying molecule in cells

Stimulates the production of nitric oxide (NO) in platelets and the endothelium
- - Endothelium - tissue that forms a layer lining blood vessels, heart, and lymphatic vessels

Nitric Oxide contributes to muscle relaxation, inhibition of platelet activation, and blood vessel dilation (vasodilation)

Red Blood Cells are the responsible component in blood to stimulate the synthesis of NO through releasing ATP

Insulin and C-peptide control the amount of ATP released by red blood cells:

Insulin and c-peptide have opposing effects on red blood cells and their production of ATP
- - Red blood cells contain PDE3, an enzyme that inhibits ATP production
When insulin binds to its receptor on red blood cells, PDE3 is activated
When c-peptide binds to its receptor (GPR146) on red blood cells, PDE3 is inactivated

When PDE3 is inactivated: red blood cells release excess ATP
- - Since it is an inhibitor of ATP production, inactivating PDE3 promotes the release of ATP
ATP triggers the activation of ENOS (an enzyme that produces nitric oxide) in endothelial cells
- - Endothelial cells - cells that make up the endothelium

Together, insulin and c-peptide adjust how much ATP is released by RBCs, controlling the production of NO, which affects vasodilation.

Modeling this Process:

Click the arrows to adjust the amount of insulin and c-peptide on the model.

Watch how changing these amounts affects PDE3 activation, ATP release by red blood cells, ENOS activation, production of nitric oxide, and vasodilation.

What is Vasodilation?

Vasodilation occurs when the muscles lining arteries relax, dilating blood vessels
- - Lowers blood pressure
  - Increases blood flow
  - Allows more oxygen into the blood

How does Vasodilation work?

As red blood cells travel through constricted blood vessels, they are put under physical stress
This stress causes the red blood cells to release ATP
- - Insulin and c-peptides are necessary to do so
The release of ATP triggers the production of NO
NO relaxes the endothelial cells
This relaxation dilates the blood vessels
As blood vessels expand, stress on red blood cells is reduced

This process is a negative feedback loop:

Click the top arrow to increase the stress put on red blood cells by blood vessel constrictions. Watch how vasodilation decreases this stress.

Now, pretend this is occuring in a Type 2 Diabetes patient. Increase the insulin resistance. Watch how this inhibits vasodilation.

Since vasodilation depends on insulin and c-peptides to trigger NO production, a patient with diabetes has decreased vasodilation, which can lead to thrombosis (blood clots).

High blood glucose levels & inhibition of vasodilation from diabetes leads to atherosclerosis.

What is atherosclerosis?

Atherosclerosis is a disease of the arteries where plaques of fatty material are deposited on the arteries's inner walls. This causes the hardening and narrowing of the arteries, and can put blood flow at risk as the arteries become blocked. Atherosclerosis can lead to increased risk of heart disease & stroke.

There are 3 main stages of atherosclerosis:

The first stage of atherosclerosis, the fatty streak, is when a yellow streak appears along major arteries, such as the aorta and carotid artery. This streak is made up of:

smooth muscle cells
cholesterol
and macrophages (a type of white blood cell)

The fatty streak phase does not present any noticeable symptoms, but can progress into a more dangerous phase of atherosclerosis: the fibrous plaque.

The second stage of atherosclerosis, the fibrous plaque, is when a fibrous plaque develops within the inner layer of the artery. This plaque is made up of:

smooth muscle cells (contain cholesterol inside of them)
macrophages
and lymphocytes (a type of white blood cell)

As the fibrous plaque continues to grow, it protrudes into the vessel where the blood is flowing.

The final stage of atherosclerosis, the complicated lesion, is when the fibrous plaque breaks apart, exposing the cholesterol and connective tissue underneath. This process is recognized by the body as an injury, causing a team of blood clotting cells to be sent to the scene. The blood flow is now being restricted by a complicated lesion, or the ruptured plaque in combination with the blood clot that has formed.

What happens when one has atherosclerosis?

Narrowing of arteries leads to lack of blood flow/circulation throughout body
- - If circulation to hands & feet is cut off, one may experience intermittent claudication
    - - Intermittent claudication - a condition where cramping and/or pain in the leg is induced by exercise, typically caused by obstruction of the arteries
  - Poor circulation can also cause peripheral neuropathy
    - - Peripheral neuropathy - a type of diabetic neuropathy that causes decreased sensation in the extremities
      - may prevent one from noticing an injury or infection, such as an ulcer in the foot

What is the Glucose-Insulin Regulatory System?

The Glucose-Insulin Regulatory System is a dynamically active system in which insulin and glucose levels are maintained within a narrow ideal range. It is maintained by feedback between glucose and insulin levels with various tissues in body.

Elements of the Glucose-Regulatory System:

Pancreas
Liver
Muscles
Fat Cells

Pancreas's Effect & Purpose:

Secretes digestive enzymes which break down carbohydrates, fats, proteins in food entering small intestine from stomach
Endocrine component of pancreas is located within regions of pancreas called pancreatic islets of Langerhans
- - When blood sugar increases, beta cells in pancreatic islets secrete insulin
  - When blood sugar decreases, alpha cells secrete glucagon
  - Delta cells secrete somatostatin to help regulate production of insulin & glucagon
  - Epsilon cells produce ghrelin which affects feelings of hunger
  - Pancreatic polypeptide cells (PP cells) produce pancreatic polypeptide which helps regulate pancreatic and gastrointestinal secretions & functions

What happens when blood glucose gets too high?

beta cells in pancreas secrete more insulin = liver & muscles store excess glucose by converting it to glycogen
- - Glycogen - glucose molecules branched together to store glucose for later use
    - - Stored in liver & skeletal muscles, some in heart & brain
  - Can also store glucose as triglycerides
    - - Stored in adipose tissue (a type of loose connective tissue)

Insulin's Effect & Purpose:

Primary hormone that lowers blood glucose by increasing glucose uptake & utilization by liver, muscles, fat cells
- - Affects carbohydrate, fatty acid, and amino acid metabolism
  - Stimulates glycogenesis (synthesis of glycogen by uptake & storage of extra glucose)
  - Prevents breakdown of glycogen by inhibiting glycogenolysis
Insulin causes fat cells to uptake glucose and convert excess glucose into fat through a process called fatty acid synthesis
- - Fatty acids are converted into triglycerides and stored in adipose tissue as fat
Insulin forces cells to absorb circulating amino acids and decreases the breakdown of proteins

Glucagon's Effect & Purpose:

Glucagon affects carbohydrate, fatty acid and amino acid metabolism
- - Stimulates the breakdown of glycogen (stored carbohydrates) to be released into the blood as glucose, which is a process called glycogenolysis
  - Stimulates the breakdown of stored fat (triglycerides) into fatty acids for use as fuel by cells (lipolysis)
  - Stimulates the breakdown and conversion of amino acids into glucose, in a process called gluconeogenesis

Activity - Exploring the Effect of Diabetes on the Glucose-Insulin Regulatory System:

Compare and contrast the 3 Sage Modeler Diagrams linked below:

1. Normal Glucose-Insulin Regulatory System

2. Glucose-Insulin Regulatory System of Type 1 Diabetes Patient

3. Glucose-Insulin Regulatory System of Type 2 Diabetes Patient

Click on the "simulate" arrow at the top of each of diagram screen and take a moment to increase & decrease the levels of different node values. Consider the following questions:

How do the effects of increasing/decreasing values differ between models?
In what ways is the change seen consistent with the previous information provided about the glucose-insulin regulatory system?

Skeletal muscle is where most of the body’s glucose uptake occurs:

Glucose uptake is stimulated by insulin and muscle contraction
During exercise, insulin is not required for glucose uptake in muscles because it causes muscle contraction
- - However, insulin does increase glucose uptake
  - Patients with Type 2 diabetes have a decreased uptake due to the lack of working insulin

Diabetes decreases ATP production:

C-peptide, which is formed during insulin processing, is essential in the release of ATP from red blood cells
Patients with diabetes are either insulin resistant or unable to process insulin
- - These patients lack the necessary c-peptide
ATP is vital to many biological functions as it provides energy to cells
- - As diabetes inhibits ATP production, it can have a major effect on different cell processes

Type 2 Diabetes affects vasodilation in blood vessels:

Normally, insulin increases ENOS activation in endothelial cells, causing vasodilation
Type 2 diabetes patients are insulin-resistant, therefore, their endothelium-dependent vasodilation is reduced

Diabetes can negatively impact the circulatory system:

Diabetes raises risk of high blood pressure, putting a further strain on the heart
Since diabetes inhibits vasodilation, patients with diabetes are less able to combat atherosclerosis

Narrowing of arteries, or atherosclerosis, leads to poor blood circulation/flow

Effect of Diabetes on the Glucose-Insulin Regulatory System:

In Type 1 diabetes patients, the beta cells in the pancreas are unable to produce insulin which prevents blood glucose levels from going down, therefore leading to the body's inability to maintain homeostasis
In Type 2 diabetes patients, the body is unable to use insulin the way it should, preventing blood glucose levels from lowering similar to the effect of Type 1 diabetes

For people with diabetes, insulin can be injected via a needle into the fat underneath skin to reach the bloodstream:

Medical insulin has 3 characteristics:
1. 1. Onset - amount of time until insulin reaches bloodstream and starts lowering blood sugar
  2. Peaktime - time when insulin is in its maximum strength of lowering blood sugar
  3. Duration - how long insulin continues to lower blood glucose levels

There are different types and strengths of insulin
- - Types are based on the rate at which they work, when they peak, and the duration of their effect
  - The most common strength is used in medicine in the United States is U-100

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Project Made in July-August 2020

The content of these pages was created by students for students with the help of educators and scientists who were funded by National Science Foundation Award DBI-1565166. The views expressed herein are those of the authors and do not necessarily reflect the views of NSF or ISB.

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