EAST 501 Lecture Notes - Lecture 17: C-Peptide, Disulfide, Paracrine Signalling
21 Jun 2018
School
Department
Course
Professor
Feb 19th – Lec 17 Bernard
Diabetes
OUTLINE
- Glucose homeostasis and insulin/glucagon action
- Type 1 diabetes and treatments
à
treat just with insulin (only option)
REGULATION OF GLUCOSE HOMEOSTASIS BY INSULIN AND GLUCAGON
- Type I diabetes – just treat with insulin
- Prandian state = around meal time
- meal à food into gut à increases glucose into blood à want to move glucose out of the blood and into the
tissues (fat, skeletal muscle, liver) à moving glucose out is dependent on insulin
- the pancreas has islets à the islets are filled with different endocrine cells (including) beta cells à these beta
cells detect changes in insulin levels and in response will secrete insulin à insulin will act through its receptor at
the different tissues and stimulates those cells to uptake glucose from the blood (want 6-7mmol/L in the blood)
o 7mmol/L is the magic number when diagnosing
o pancreas is always what is detecting when the glucose levels are either really low or really high
- the tissue that demands the most glucose is the brain
- the glucose uptake in the brain is different compared to the rest of the body
o it is not insulin dependent
o you have constitutive expression and localization of different glucose transporters in the brain
o you want to have enough sugar in the blood to feed the brain
- when you are in a fasted state (sleep is a fasting state) à when waking up there is a circadian rhythm with
glucocorticoids à cortisol increases during waking as part of the circadian rhythm à stimulates liver to produce
sugar à sugar feeds brain
- pancreas has alpha cells (next to beta cells) that make glucagon
o glucagon does the opposite of insulin à stimulates the liver to synthesize glucose and secrete into the
circulation
o if not enough glucose in the circulation we need our tissues to produce à gluconeogenesis (sugar
production in body)
Feb 19th – Lec 17 Bernard
Diabetes
ISLETS OF LANGERHANS
- mice – islets ARE FILLED with beta cells that are making insulin (overabundance of beta cells and fewer of the
other cell types)
o when using mouse as a model (in drug development) à need to take into consideration that the islets
are different in structure and composition, compared to humans
- human pancreatic islets contain fewer beta cells and a higher number of alpha cells compared to mouse islets
o in humans there is a more equal number of alpha and beta cells in the islet
o human alpha, beta and delta cells are intermingled throughout the islet with no particular order of
distribution along islet blood vessels
- delta cells secrete somatostatin
- there is important paracrine regulation in the islet
o alpha cell à contain receptors for insulin and somatostatin à allows for paracrine signaling à play a
role in the secretion of glucagon
- there is an intimate relationship between the cells and the vasculature in the islets
INSULIN BIOSYNTHESIS AND PROCESSING
- made as a pre-pro hormone
o at the N-terminus there is a signaling peptide
that directs it to the lumen
of the ER during translation where it gets cleaved
- preproinsulin composed of B chain, C peptide and A chain = linear
o in ER lumen à folded
o 3 disulfide bonds are formed
- one parent molecule à generation of 3 molecules
- there are cleavage sites for pro-hormone convertases
o PC1 and PC2 à cut the precursor protein into A, B and C chains
- there are 2 disulfide bonds between the A chain and
the B chain
o there is also an intra-molecular disulfide
bond in the A chain
o A and B are no longer attached to the C peptide
- results in dimeric ligand of the A and B chain (disulfide linked)
and C peptide
- be aware of C peptide à when insulin is secreted C peptide is also secreted at an equal molar amount
o you can measure the C peptide and that will give you an indirect measure of how much insulin was
secreted
o it does have biological activity à can bind to the membranes of several tissues and has a biological
function
o needed for Proinsulin = allows proper formation
PRIMARY STRUCTURE OF THE HUMAN INSULIN
- there are differences in the a chain between the cow, pig and human
- there are aa differences between the species à for that reason à
can’t use porcine/bovine insulin sources à would have immunogenic
responses if you tried to use bovine or porcine insulin à
use recombinant insulin instead (but it is the human form)
HOW IS INSULIN RELASED REGULATED? – the big one is in response to blood glucose levels – there is a nervous system
and endocrine interplay in the regulation of insulin
- Autonomic NS à there is neural innervation of the islets themselves
o sympathetic inhibition (adrenergic inhibition)
B30
A8
A10
HUMAN
Thr
Thr
Ile
PORCINE
Ala
Thr
Ile
BOVINE
Ala
Ala
Val
Feb 19th – Lec 17 Bernard
Diabetes
o parasympathetic stimulation
§ cholinergic stimulation
- incretin hormones (GLP-1, GIP) – gut-derived hormones
o somewhat similar to glucagon
o some therapies for type II are based on the role of these incretins
- paracrine regulation in islets
- blood glucose levels
GLUCOSE REGULATION OF INSULIN RELEASE
- there are different family members of glucose transporters – GluT1 and GluT2 (at the membrane of beta cells)
o these are usually multiple TM-spanning proteins
o they have a pore à allow certain molecules to pass through them (in this case they have specificity for
glucose)
- once passed through the beta cell, glucose is subjected to glycolysis à it is broken down
o the end products of glycolysis: pyruvate and ATP à making energy
o pyruvate goes into the mitochondria, into the Krebs cycle and more ATP is made
o converting sugar into energy
- through glycolysis there is an increase in the ATP in the cell
- there are ion channels that can sense the ATP concentration (K channels are sensitive to ATP)à when the ATP
concentrations reach a threshold some of the channels close à there is an accumulation of these positively
charged potassium ions in the cell à this changes the polarity of the cell
o when kicking out the cation = hyperpolarized state (not excited)
o accumulation of cation = changed to a depolarized state
- there are other membrane channels that are sensitive to voltage à when the cell is depolarized à opening of
voltage gated Na and Ca channels (particularly interested in the Ca channels) à the opening of Ca channels
leads to influx of Ca à stimulates the release of insulin from vesicles (insulin in the readily releasable pool)
Document Summary
Type 1 diabetes and treatments treat just with insulin (only option) Regulation of glucose homeostasis by insulin and glucagon. Type i diabetes just treat with insulin. Made as a pre-pro hormone: at the n-terminus there is a signaling peptide that directs it to the lumen of the er during translation where it gets cleaved. Preproinsulin composed of b chain, c peptide and a chain = linear in er lumen folded: 3 disulfide bonds are formed. The big one is in response to blood glucose levels there is a nervous system and endocrine interplay in the regulation of insulin. Autonomic ns there is neural innervation of the islets themselves: sympathetic inhibition (adrenergic inhibition) Bernard incretin hormones (glp-1, gip) gut-derived hormones: somewhat similar to glucagon, some therapies for type ii are based on the role of these incretins. Insulin stimulates glucose uptake via regulation of glut4.
