Watson - Diabetes Flashcards
How was the 1st disease model of diabetes created?
- studied lipid metabolism and suspected pancreas played a role → removed pancreas of dogs and saw exhibited symptoms of diabetes
What is insulin?
- polypeptide hormone that controls circulating levels of glucose
Where is insulin synthesised?
- in pancreatic β cells as preproinsulin
How is insulin cleaved?
- proteolytically cleaved to gen 2 chains joined by disulphide bonds
What is the structure of insulin?
- 110 res
- chain A and B
What is the corresponding opposing hormone to insulin?
- glucagon
What cells release glucagon?
- pancreatic alpha cells
What are the characteristics of TID?
- typically adolescent onset, “juvenile diabetes”
- cachexia
- absolute req for insulin
What are the characteristics of TIID?
- typically mature onset
- may not req insulin
- can be mod by diet/exercise
- not always due to obesity, big genetic component
What are the key tissues involved in glucose homeostasis?
- pancreas
- liver
- muscle
- adipose
What is the typical glucose blood level?
- 5mmol/L
- 2 hrs after meal can increase to 8mmol/L (then decreases again)
How do endocrine tissues release hormones?
- release them into circulation by diffusion
What are the components of pancreatic beta cells?
- Kir 6.2 = ATP-gated inwardly rectifying K+ channel
- L-type Ca2+ channel = voltage dependent
- GLUT2 glucose transporter
- Hexokinase IV (glucokinase)
What happens when glucose enters beta cells, and what is the role of ATP?
- enters via GLUT-2 (inefficient glucose transporter)
- phos by hexokinase IV
- G-6-P metabolised by mt, alt levels ATP in cell
- can’t leave as charged
- ATP and other oxidative metabolites act as sensor for circulating glucose levels
- increased ATP closes K+ transporter, causing -ve shift in cell pot
- voltage shift activates Ca2+ channel –> Ca2+ influx
Ca2+ influx drives membrane fusion of secretory granules and release of insulin
What kind of receptor is the insulin receptor and where is it present?
- RTK
- present on basically all cell types
What happens when insulin binds its receptor?
- receptor phosphorylated –> activating S/T kinases
- leads to phos of series of insulin receptor substrates (IRS-1 to IRS-4)
- phos of IRS-1/2 activates phosphoinositol pathway –> activates PI3K, which activates PDK1
- then phos and activation of Akt (prot kinase B)
What is the role of Akt, (ie. why is it the main mediator of insulin)?
- Akt signalling drives cytoskeleton rearrangements that lead to insertion/activation of high affinity transporter GLUT-4 –> increases uptake of glucose 20x
- increased uptake by skeletal muscle and adipocytes decreases blood levels quite rapidly –> these tissues act as reservoir for excess glucose, eg. after meal, v important
What are the effects of insulin in the liver?
- downreg gluconeogenesis
- upreg glycogenesis (if excess glucose want to store it)
- downreg glycogenolysis
- upreg lipogenesis (makes fats as LT store of energy)
What are the effects of insulin in adipocytes?
- increased glucose transport
- increased lipogenesis (fat prod)
- decreased lipolysis (fat breakdown)
What are the effects of insulin in the pancreas?
- decreased glucagon levels
- increased β cell growth
What are the effects of insulin in skeletal muscle?
- increased glucose transport (take up glucose from blood)
- increased glycogenesis (glycogen made as store of energy)
What is glycogenesis and why is it needed?
- synthesis of polymer of glucose
- way of storing glucose as branching polymer
What is glycogenolysis?
- opp process to glycogenesis
- breakdown of glycogen in liver to release glucose when levels too low
When is lipogenesis needed and what is it?
- in times of glucose sufficiency
- acetyl-CoA used to synthesise FAs
- FAs esterified w/ glycerol to form triglycerides
- some tissues can use FA, eg. adipocytes, liver, but not brain
What is lipolysis?
- opp process to lipogenesis
- breakdown of triglycerides to prod FFA, in periods of glucose depletion
What is glucagon?
- 29 AA peptide hormone w/ opp actions to insulin
What effects does glucagon have?
- increases gluconeogenesis and glycogenolysis
- decreases lipogenesis
What is the cause of TID?
- problem in thymus, where T cells dev, so recognise self as non self cells, so destroy β cells
- ‘starvation in presence of plenty’
How do insulin and blood glucose levels change t/o lifetime of diabetic?
- DIAG*
- early stages characterised by increasing blood glucose levels and often asymptomatic
- insulin levels become abnormally high as pancreas responds to glucose
- later insulin levels progressively fall as β cell mass and productivity fall off –> hyperglycaemia
What causes TIID, ie. the initial event?
- not responding to signal –> insulin sensitivity, then loss of insulin prod/beta cells
What is the pathogenesis of TIID?
- initial event is loss of response to insulin signalling –> ie. ‘insulin resistance’
- glucose remains high –> driving increased insulin prod by β cells
- initially increased insulin prod maintains normal glucose levels
- β cell mass and productivity fall away and insulin levels progressively fall until glucose levels uncontrolled
When do TIID patients req exogenous insulin?
- typically after 7-8 yrs
When is TIID usually diagnosed?
- after insulin production has fallen sufficiently that blood glucose levels give rise to symptoms of hyperglycemia
What glucose levels would a pre-diabetic have?
- > 7 mmol/L (fasting) and >11 mmol/L (random)
What are glucose meters and how do they work?
- engineered forms of glucose oxidase coupled to test strip
- FAD-glucose deHase used –> metabolises glucose
- chemical e- acceptors contained in electrode re-ox FAD
- gen electrical current
- damages small blood vessels (capillaries, capillary wall integrity and neuropathy, proteins become brown)
What is the aim of glucose meters, and why are they not ideal in this respect?
- mimic natural control
- would be better if monitoring constantly
What is the pathogenesis of TIID on adipose tissue?
- lipolysis not inhib by insulin (as not sensitive to insulin)
- so increase in FFA into circulation, as breaking down fats
What is the pathogenesis of TIID on the liver?
- processes FFA to increase triglycerides
- increases lipid in blood in form of LDL/HDL (can lead to atherosclerosis, hence its assoc w/ TIID)
- gluconeogenesis can be active even w/ high circulating glucose concs (liver thinks shortage of glucose)
Which type(s) of diabetes cause hyperglycemia?
- both
What are the effects of hyperglycemia?
- polyuria (= excess urine)
- neuropathy (= damage to nerves) –> esp in periphery
- macrovascular damage (big vessels)
- microvascular damage (small vessels)
- infection
What happens in the kidney during hyperglycemia?
- above renal threshold (11mmol/L glucose) not enough active transport to take all glucose back in
- so overwhelms transport system and start passing glucose into urine
- so filtrate contains glucose, exerts osmotic pressure, so reduced water absorption = increased vol of urine and thirst
- then damage to glomerulus, damages kidney, can fail, prots secreted, attacks tubule lining, inflam, glucose itself is irritant and renal failure
What is glucosuria?
- glucose in urine
What are the microvascular complications assoc w/ diabetes?
- high glucose toxic to endothelial cells lining capillaries
- induces expression of fibrinogenic GFs (means prod more ec matrix)
- -> PDGF (platelet derived GF)
- -> TGFβ (transforming GF β)
- increased dep of matrix in basement membrane (much more drastic effects in kidneys than capillaries, but still issue)
- thickening of vascular wall and loss of elasticity in vessel (can’t stretch in response to BP)
- loss of circulation (due to narrowing of vessel), mainly in periphery and ischemia
What are the macrovascular complications assoc w/ diabetes?
- pathological effects of high glucose concs increase permeability of endothelial cell lining of arteries
- liver now prod lots of LDL, so lots in circulation, can enter underneath vessel due to increased permeability, so deposition of lipids in supportive layers of vessels = atheroma (plaque)
- restricted blood flow can lead to coronary artery disease → approx 2x increase risk
What are the results of hyperglycemia leading to infection in diabetes?
- high levels of tissue glucose provide good habitat for MOs
- reduced microvascular function changes nature of normal inflam response (so infection can go unnoticed) –> reduced diffusion of normal inflam mediators and reduced migration of immune cells
- peripheral neuropathy can reduce awareness of damage (can get out of control more quickly, amputation etc.)
What role does β cell depletion play in the pathogenesis of TIID?
- glucotoxicity = persistent high levels glucose cause apoptosis and loss of β cells
- FA = high levels circulating FA assoc w/ glucose insensitivity
What role does amyloid play in the pathogenesis of TIID?
- β cells in advanced TIID show presence of abnormal prot aggregates, lead to LoF and cell death
- -> aggregates formed from normal cellular prot, IAPP (islet amyloid polypeptide)
- -> IAPP co-expressed w/ insulin, shares common promoter regulatory motifs
- -> excess levels of insulin prod in early stages might be driving prod of amyloid, which is ultimately killing β cells
What is the concordance in both types of diabetes?
- TID MZ concordance = 30%
- TIID MZ concordance = 40-100%
What genes has TID been assoc w/?
- HLA, DR3 and DR4
How is genetic and env basis of TIID diff?
- stronger genetic basis, but also dep more on env
Why is TIID becoming more common?
- due to western affluent lifestyle and dietary habitats
What did a study on Pima indians show about TIID
- when migrated and changed lifestyle had greatly increased incidence of TIID, despite genetic background being the same
- therefore dep on env
What genes are involved in TIID?
- Calpain 10 = protease, assoc w/ insulin release from pancreas cells
- Glucokinase = hexokinase IV, glucose sensing
- Glucose transporter, GLUT-2 –> transports glucose into cell, gene variants may disrupt glucose reg
When do the genes involved in TIID diabetes become important?
- when lifestyle puts them at risk
What interventions are there for TIID
- Sulfonylureas
- Metformin
- insulin
How does sulfonylureas work as an intervention for TIID?
- targets SUR1 subunit of ATP gated K+ channel, blocking K+ transport
- triggers Ca2+ influx and increased insulin secretion
How does metformin work as an intervention for TIID?
- mech unclear, but main target organ is liver
- passes unmetabolised through body
- alt membrane fluidity
- increases insulin receptor signalling sensitivity, reduces gluconeogenesis in liver and increases GLUT-4 activation and improves glucose transport
- can be good at start of disease as increases insulin sensitivity, so easier to control glucose levels
What do those w/ TID req for survival?
- exogenous insulin
Why is TID described as ‘starvation in the midst of plenty’?
- insulin essential for uptake and utilisation of glucose
- but complete destruction of pancreatic beta cells –> no insulin prod and glucose not utilised
How does a TID patient typically present?
- wasting of muscles and other tissues
- polyuria = excessive urine
- glycosuria = excess glucose in urine
- polydipsia = excessive thirst
How is TID an eg. of an organ specific autoimmunity?
- no. of diff high affinity autoantibodies against beta cell prots are detected in patients
- can be detected in children from 9 mo
- detection usually forecasts dev of TID in adolescence
- evidence for presence of islet specific CD4 T helper cells (somatic hypermutation –> T cells stim autoreactive B cell dev)
- patients exhibit ‘insulinitis’ even before symptoms evident (infiltration of lymphocytes in pancreas)
- beta cells silently destroyed and insulin prod fades
- diagnosis only when symptoms appear
What high affinity auto Abs against beta cell prots are detected in TID patients?
- glutamic acid decarboxylase (GAD)
- insulin (IAA)
- Zn transporter 8 (ZnT8)
- insulin assoc antigen 2 (IA-2)
What can the genetics of TID tell us?
- understanding risk factors can lead to general insight into pathogenesis of other autoimmunities, screening for those at increased risk and suggestions of ways to intervene in early stages
What did initial case/control studies of TID involve?
- early studies focussed on HLA region –> assoc was strong enough to prove
- limits of tech, serotyping of HLA, SB of gDNA, compare alleles in 100-200 indivs w/ and w/o TID
What is the problem w/ case/control studies in TID?
- lacks power as no.s limited, genetic diversity between pops dilutes effect of risk alleles, often results not reproducible
How much of the TID risk do HLA alleles account for?
- 40-50%
Which class of HLA alleles are involved w/ disease?
- no. of specific HLA class II alleles are known to be assoc w/ increased risk of disease
- recently high res linkage studies have also estab assoc w/ HLA class I alleles
Why might HLA alleles be involved w/ TID?
- viral infection and IR to them play signif part in triggering autoimmunity to islets
What are the advantages of the NOD mouse as a model in TID?
- well characterised phenotype and genetics
- no. inbred strains
- reduced genetic variability - enables powerful genetic analysis
- easy to breed large no.s
What is a NOD mouse?
- non obese diabetic mouse
- spontaneously dev TID in 90% females
- mirrors human disease: insulinitis, autoantibodies, progressive loss of beta cells
How was assoc of genes w/ TID confirmed in NOD mice, and what was discovered?
- hypothesis = risk loci identified are likely to be reflected in humans
- by crossing to other mouse strains was poss to identify genetic loci that determined susceptibility to TID
- initially assoc of HLA region was confirmed (MHC in mice)
- other loci of lesser effect rapidly identified –> approx 20, inc Idd-1, -2, -3, -4, -5, -18
What is Idd-1?
- MHC region gene
What is Idd-2, and its role in TID?
- insulin gene
- VNTR (= short stretches of DNA that are repeated in tandem)
- diff alleles w/ differing no. of repeats, some alleles protective, some increase risk, reduced exp of insulin assoc w/ increased risk (risk alleles assoc w/ reduced exp have been demonstrated in thymus)
What is Idd-3, and its role in TID?
- IL-2 gene
- IL-2 is key reg cytokine for T cells, necessary for gen and survival of TREGs (suppressive T cells)
- studies suggest NOD Idd-3 mice possess less effective TREG pops
What is Idd-4, and its role in TID?
- IFN viral response gene
- locus assoc w/ aberrant overexp of IFN response to viral infection, implications for role of viral infection in triggering in TID?
What is Idd-5, and its role in TID?
- CTLA-4 gene
- CTLA-4 is key reg of T cell antigen response, TCR activation req MHC and co-receptors, some co-receptors +vely drive T cell response, others exert -ve effect, CTLA-4 acts to dampen TCR response
What is Idd-18, and its role in TID?
- PTPN22 gene
- PTPN22 is a tyr phosphatase, exp mainly in lymphocytes, important in shutting down tyr kinase pathways activated by antigen binding in B/T cells
How have many NOD risk loci been confirmed?
- in human genetic analyses
- can do GWAS, physical maps of SNPs, fine mapping, linkage studies, sequencing, identification of gene mutations
What is CTL-4 a signif risk factor for?
- no. of human autoimmune conditions
- inc TID, autoimmune thyroid disease, rheumatoid arthritis
What is PTPN22 a signif risk factor for?
- TID and autoimmune thyroid disease
Is there evidence for role of viral infection in TID?
- evidence is dev for role of IFN antiviral response gene in human TID
- also identified protective alleles, so evidence accum
In theory how could islet transplantations be carried out for TID?
- beta cells cultivated artificially and transplanted into patients
- 1° beta cells or SC derived
- infused into liver
- beta cells able to sense blood glucose and respond approp
- better glucose control than exogenous insulin
- still experimental only
What problems are there w/ islet transplantations?
- still autoimmune, beta cells destroyed again over time
Why is the use of immunosuppressants not a viable option for treating TID?
- dangerous side effects w/ LT use, infection, tumour dev → we need the adaptive IS
Could artificial induction of tolerance be used to treat TID?
- introd high doses of antigen in absence of ‘danger signals’
- no 2nd signals of infection
- results in anergy (= absence of normal immune response) in antigen specific T cells
- experiments ongoing in animal studies
- promising results w/ insulin as antigen
- no data in humans
If dev of autoreactive T/B cells in repertoire is cause of autoimmune disease (and TID), is there a way of starting over?
- in lymphoma patients lymphopoietic SC harvested from bone marrow
- reintrod after intense chemo (completely eradicates IS) to try and reestab IS
- diversity of B/T cells unaffected and full repertoire dev again
- MIST is intl trial assessing LT benefits of SC transplants in MS (disease driven by autoreactive T cells)
- patients w/ MS shown remarkable degree of remission following transplants –> immune repertoire appears to have been reset, pot a new and effective treatment
What are the problems if reset the adaptive immune repertoire as a treatment for TID?
- will autoreactivity reappear?
- high risk of infection during treatment period, pot fatal if transplant doesn’t take
- in case of TID insulin treatment exists (there is an alt), can this approach be justified?
