L39 - Biochemical basis of Diabetes Mellitus I Flashcards
Describe function of signal peptide and post-translational modification of preproinsulin?
Signal peptide guides translocation into lumen of ER
ER:
- Disulphide bond formation + Proteolytic cleavage of signal peptide
Golgi:
- Further proteolytic cleavage
- Secretory vesicle
Describe the secretion of insulin in B cells?
Glucose enters β cells through GLUT-2
> > glycolysis and TCA cycle (oxidative phosphorylation) in mitochondria to generate ATP
> > inactivates K+ channel protein
> > membrane depolarization
> > opens Ca2+ channel, influx of Ca2+ cause insulin vesicle exocytosis
Define the locus and variability of HLA genes.
MHC/HLA locus = Chromosome 6
Highly polymorphic
Describe how HLA haplotypes can influence the risk of Type 1 DM?
MHC haplotype divided into:
1) Susceptibility alleles = increase occurrence of Type 1 DM
2) Neutral alleles = no difference in occurrence of Type 1 DM
3) Protective alleles = Lower risk of Type 1 DM
Which region of the MHC molecule is most affected by polymorphism?
Antigen binding site
Describe the role of MHC in T cell development?
1) Positive selection: Immature T cells that can bind to self-peptide/ MHC complex on thymic epithelial cells are selected
2) Negative selection in thymus medulla: MHC-restricted T cells binding to self-antigen/ MHC with very high affinity selected against
Describe how the MHC molecules can lead to release of autoreactive T cells to periphery? **
1) MHC-II polymorphism = change peptide binding region = faulty selection of T cells = failure to eliminate T cells that bind to self-antigen close to affinity threshold
» Maturation of autoreactive T cells
2) Faulty selection of Treg in thymus = compromise peripheral tolerance/ anergy of autoreactive T cells
Which gene related to glucose homeostasis is used for T cell selection and maturation?
Proinsulin gene
Describe how Proinsulin gene expression influences T cell selection?
Proinsulin gene has VNTR (variable number of tandem repeats) segment in promoter region
- If mostly class 1 VNTR is expressed in thymus medullary epithelial cells
> > Less proinsulin gene expressed
> > Less insulin made for use as self-antigen to educate T cells
> > Faulty selection of autoreactive T cells and Treg
Describe the VNTR of proinsulin expressed in pancreas?
Pancreas: Express equal amount of Proinsulin with class I and III VNTR
Compare the effects of Class I and III VNTR in proinsulin gene?
Class I = under-expression of insulin = faulty selection of Treg and T cells = increase risk of IDDM
Class III = Increase expression of insulin for T cell selection = Protective against IDDM
List 3 genes apart from proinsulin gene that are implicated in Type 1 DM development?
PTPN22 (protein phosphotyrosine-specific phosphatase)
AIRE (autoimmune regulator)
MODY
Describe the function of AIRE in T cell selection?
AIRE = transcription factor:
regulate expression of peripheral self-antigens in thymic epithelial cells for deleting autoreactive T-cells
Describe how virus can cause beta cell death in type 1 DM?
Virus infect Beta cell
» express protein similar to self-reactive peptide (autoantigen)
» Beta cell express viral antigen on MHC-II and MHC-I:
a) activate autoreactive CD4 T cells»_space; Activate B cells to make anti-B cell antibodies
b) Activate CD8+ T cell
> > Beta cell elimination
Describe how damage to Beta cells (e.g. due to diet, trauma, toxins) can lead to beta cell depletion in Type 1 DM?
Stimuli damage Beta cells
> > damaged beta cells release beta cell proteins (autoantigens)
> > APC acquire and present beta cell autoantigens on MHC-II
> > Activation of autoreactive CD4+ T cell:
- Secretion of TNFα, IFNγ»_space; activates macrophages (secrete IL-1)
- Activation of CD8+ T-cells
Explain the physiological role of MODY2?
Code for Glucokinase:
- Used for rate limiting reaction: Glucose»_space; G 6-P
- Act as threshold and determine level of glucose to trigger insulin release
Explain how MODY2 mutation leads to Type 1 DM?
Inactivate glucokinase
> > loss of regulation on intracellular glucose level
> > loss of regulation on insulin secretion
Explain how MODY3: HNF-1α, MODY1: HNF-4α, MODY5: HNF-1β mutations can lead to Type 1 DM?
Mutation = faulty expression of genes encoding for protein component in TCA-cycle
> > Decrease ATP synthesis, glucose transport
> > Unable to trigger Insulin release
Explain how MIDD mutation causes Type 1 DM?
Maternally Inherited Diabetes with Deafness (MIDD):
mutation of mitochondrial genome = cannot respond glucose, make ATP
No ATP = cannot trigger insulin release
Explain how PNDM mutation causes type 1 DM?
Mutation of Permanent Neonatal Diabetes Mellitus (PNDM):
> > permanent activation of K-ATP channel
cannot depolarize beta cell
. cannot release insulin
Explain how type 1 DM causes weight loss?
Insulin = suppress proteolysis and gluconeogenesis
Depletion of insulin = unopposed increased proteolysis and gluconeogensis
> > Elevated blood glucose not used or uptake by cells (low insulin-dependent glucose transport)
> > weight loss
Explain how Type 1 DM leads to ketoacidosis? ***
Low insulin:
- Loss of inhibition on Hormone Sensitive Lipase (HSL) = Increase conversion of TAG in adipocytes to FA in circulation
- Less Acetyl-CoA converted to Malonyl CoA = loss of inhibition on FA translocation into mitochondria = Increase FA into cell mitochondria
> > Increase B-oxidation»_space; ketone body production more than elimination by TCA cycle»_space; Ketoacidosis
Explain how Type 1 DM leads to dehydration?
Increased blood glucose: Increase osmolarity in serum:
a) increase movement of water from tissue into serum
b) decrease water reabsorption in kidney
> > Polysuria
> > dehydration
List effects of IDDM on brain?
microangiopathy, cerebral vascular infarcts, hemorrhage
List effects of IDDM on eyes?
retinopathy, cataracts, glaucoma
List effects of IDDM on CVS?
atherosclerosis, peripheral vascular atherosclerosis (gangrene, infections), MI
List effects of IDDM on Kidney?
nephrosclerosis (glomerulosclerosis, arteriosclerosis, pyelonephritis)
