BMS2002 - cell signalling Flashcards
3 stages of signal transduction
- reception - EC signal activates membrane receptor
- transduction via amplifiers, second messangers, pathways
- response
responses from signal transduction
transport protein->altered ion transport
metabolic enzyme -> altered metabolism
gene regulatory protein -> altered gene expression
cytoskeletal protein -> altered shape or movement
cell cycle protein -> altered cell growth and division
first messengers are..
chemicals that can serve as extracellular signalling molecules
examples of second messengers
amines, peptides/proteins, steroids, other small molecules (aas, ions, gasses)
cell surface receptors
bind EC molecules -> wide range of IC signal transduction pathways
FAST response
intracellular nuclear recdptors
bind molecules that can freely pass membrane by diffusion
often act as transcription factors
SLOW response
4 main classes of receptors
- ligand gated ion channels
- GPCRs
- Enzyme-linked
- Nuclear
nicotinic acetylcholine receptor
ligand gated ion channel
binds Ach -> channel opens -> Na+ in
binds nicotine
electrical event -> response
calcium can also enter
y-amino buytyric acid gabaA receptor
ion channel Cl-
inhibitory receptor
activated by benzodiapines, alcohol, anaesthetics
muscarinic acetylcholine receptor
indirectly links with membrane ion channels (signal transduction pathways)
more sensitive to muscarine than nicotine
ionotropic receptors
form an ion channel pore
e.g. nictotinic Ach receptor, gabaA
metabotropic receptors
indirectly linked with membrane ion channels via signal transduction pathways - often g protein mediated
GPCR mechanism
- unstimulated cell
- adrenaline binds beta adrenoreceptor
- b adrenoreceptor - G protein interaction
- GDP/GTP exchange
- alpha subunit liberation
- Free a subunit activates AC -> camp -> PKA activation
- unbinding of adrenaline/ GTP hydrolysis -> unstimulated cell
cAMP activates
PKA
PKA..
catalyses transfer of ATP to specific serine/threonie residues on substrate proteins
physiological responses mediated by cAMP/PKA
Kidney collecting duct - activated by vasopressin -> water retention
Vascular smooth/cardiac muscle- activated by adrenaline -> relax/increase HR
Colonic epithelium- various factors -> fluid/electrolyte secretion
Pancreas - glucagon-> release of glucose into blood
b-ARK
beta adrenoreceptor kinase
steps of desensitization of receptor
- protein phosphorylation -> cell response
- PKA phosphorylates b-ARK -> increases activity
- b-ARK phosphorylates b-adrenoreceptor -> reduced affinity for adrenaline
- reduced affinity -> reduced cellular response (even if sustained stimulation)
Gs
stimulates AC
- couples with b-adrenoreceptors, vassopressin receptor, A2a/B adenosine receptors
Gi
inhibits AC
- a2 adrenoreceptors, m & a opiod receptors, A1/3 adenosine receptors
how does cholera toxin target G-proteins
cholera toxin activates a s subunit -> ADP dephosphorylation -> conformational change to cause permanent binding to GTP -> constant signalling -> continuous cAMP production
- in colon: activation of PKA-dependent Cl- channels -> lose water and Na+/Cl-
-> secretory diarrhoea
how does pertussis toxin affect g-proteins
acts on alpha I subunit -> forces subunit to remain inactive -> prevents activation of AC/PKA -> phosphorylation -> symptoms of whooping cough
Gq pathway
Gq stimulates PLC -> cleaves PIP2 into IP3 and DAG
IP3 -> Ca2+ release from ER
DAG-> recruits PKC -> regulates cell shape/proliferation/ transcription factors, mediates desensitization
blood pressure control
alpha1- adrenoreceptor -> vasoconstriciton -> increase blood pressure
beta2-adrenoreceptor -> vasodilation -> decrease blood pressure
Receptor Guanylyl Cyclases mechanism of signalling
ANP binds -> receptor dimerization and activation -> generates cGMP -> activates other signalling molecules
Receptor serine/threonine kinases mechanism of signalling
first messenger binds to receptor type 2 -> receptor 1 then binds, forms complex with all three units -> type 2 phsphorylates type 1 -> activates ser-thr kinase activity of type 1 -> type 1 phosphorylates target proteins
Receptor tyrosine kinases (RTK) mechanism of signalling
binding of two insulin molecules -> receptor dimerises -> phosphorylate eachother -> creates phosphotyrosine motifs -> recruit intracellular signalling molecules
Tyrosine kinase-associated receptor mechanism of signalling
first messenger binds -> dimerization of the receptor -> activation of tyr kinases -> these phosphorylate tyr residues on themselves and on the receptor -> these motifs recruit intracellular molecules
Receptor tyrosine phosphatase mechanism of signalling
CD45 binds -> activates tyr phosphorylase activity -> dephosphorylates target protein -> regulation of downstream cell-signalling events
how is glucose stored?
as glycogen, mostly in muscles and liver
after a meal
increased glucose absorption -> increased glucose conc in circulation -> can stimulate metabolism and increase o2 demand
abundant glucose stored as glycogen
between meals
absorption is minimal -> reduced glucose conc -> may limit metabolism and reduce o2 demand
pro-insulin
converted by prohormone convertase -> removes C peptide -> left with 2 polypeptide chains held by disulphide bridges
two phases of insulin secretion
first: release of insulin stored in secretory granules (rapid, transient)
second: synthesis and secretion of new insulin (slower, sustained)
insulin circulates..
freely - v little plasma protein binding
insulin is degraded by
insulinase, mainly in liver/muscle/kidneys
Glucose transporters on beta cells
GLUT2
- system is hormone-insensitive (always active)
- glucose -> ATP
beta cells K+ channels
ATP sensitive - open at normal levels, closed at very high levels of ATP
-> control membrane potential
-> external glucose can set membrane potential
beta cells VG Ca2+ channels
- opened by depolarisation -> makes cells permeable to calcium
-> depolarisation increases membrane permeability to Ca2+
Beta cell exposed to normal glucose
normal glucose -> normal internal ATP -> K+ channels open -> Vm is hyperpolarised -> Ca2+ channels closed -> b cell doesnt secrete insulin
beta cell exposed to high glucose
high glucose -> high internal ATP -> K+ channels close -> depolarised Vm -> Ca2+ channels open -> b cells secrete insulin
insulin is released from the (1) into the (2)
- pancreas
- hepatic portal vein
insulin receptors
dimeric: alpha and beta subunits
- insulin binding -> dimerization -> activation
- receptor dimerises -> subunits phosphorylate eachother at multiple tyrosine residues
insulin receptor signalling
insulin binds receptor -> receptor dimerization + activation -> receptor phosphorylates IRS-1 -> P13K activation -> cellular responses to insulin
insulin promoting glucose uptake by liver
insulin induces P13K activation -> P13K activates PKB -> induces translocation of GLUT4 to plasma membrane -> GLUT4 allows glucose entry
insulin promoting glycogen synthesis
insulin activates PKB -> activates GSK-3 -> activates glycogen synthase -> glycogen synthesis
insulin promotes fat deposition in adipocytes
insulin allows glucose into cell via GLUT4 -> glucose metabolised to glycerol
- insulin inactivates lipase
- excess fatty acids in the cell
insulin promotes synthesis of new proteins
insulin receptor -> P13K -> TORC1 -> protein synthesis
- when amino acids are abundant, insulin stimulates their incorporation into proteins
2 things that promote insulin release
- hyperglicemia
- raised levels of amino acids
4 effects of insulin
- promotes uptake and storage of glucose
- promotes metabolic utilisation of glucose
- promotes storage of fat
- promotes synthesis of new protein
glucagon is secreted by (1) in response to (2)
- pancreatic alpha cells
- low glucose concentration
glycogenolysis
glucose release from liver
glycogenolysis signalling pathway
glucagon activates AC -> cAMP activates PKA -> activates phosphorylase kinase A -> phosphorylase A
(adrenaline can also activate this pathway by b-adrenoreceptors)
gluconeogenesis
glucose formation from lipids and amino acids stimulated by glucagon
3 things that stimulate glucagon release
- hypoglucemia
- vigorous exercise
- raised levels of amino acids
3 effects of glucagon
- glycogenolysis
- gluconeogenesis
- ketogenesis
Diabetes mellitus
inability to regulate blood glucose
blood sugar levels in diabetes
> 7mM (5mM is normal)
pathogenesis of T1DM
environmental factors/genetic predisposition -> autoimmune destruction of beta cells (CD8+ mediated) -> severe insulin deficiency -> hyperglycaemia
genes associated with T1DM
HLA-DR3/DR4
no insulin ->
glucose not used as metabilic fuel -> rapid weight loss
-> fatty acids -> ketone bodies -> decreased pH -> metabolic acidosis -> acidic coma
hyperglycemia and dehydration
high conc of glucose enters glomerular filtrate -> overwhelms PCT glucose absorbing capacity
-> increases fluid osmolarity in tubules
-> more water secreted from cells into PCT
-> water reabsorption is reduced
-> urine flow is increased
-> dehydration, excessive urine production, thirst
problems with injecting insulin therapy
- exogenous insulin into general circulation, natural insulin into portal circulation
- lipohypertrophy (fat deposition around repeated injection site as insulin promotes fat deposit) -> unpredictable rate of insulin absorption
soluble insulin
rapid/short lived
IV in emergencies
isophane insulin
intermediate acting
tends to form precipitates
insulin zinc suspension
long acting
tends to form precipitates
insulin lispro
insulin analogue
v. rapid/short lived
normally taken before a meal
insulin glargine and detemir
insulin analogue
long acting, slowly absorbed
normally taken before a meal in combo with short-acting
forms a micro-precipitate at physiological pH
teplizumab
drug to target autoimmune reaction in diabetes type 1
pathogenesis of T2DM
- genetic and environmental predisposition (lifestyle, obesity)
- insulin resistance
- hyperinsulinemia -> beta cells try to compensate for peripheral resistance
- beta cells fail to keep up -> hypoinsulinemia
- diabetes: total failure of insulin
diabetes 2 and free fatty acids (FFA)
excess FFA are transformed into DAG -> activate PKC -> phosphorylate IRS-1 -> attenuates insulin receptor signal
-> insulin resistance in muscle and liver
diabetes 2 and adipokines
released by adipocytes
can be pro- or anti-hyperglycaemic
adiponecting (anti-hg) is reduced in obesity
diabetes 2 and inflammation
adipocytes produce IL-6 and IL-1 -> attract macrophages to fat deposits
reduction of cytokines can improve insulin sensitivity
diabetes 2 and PPARy
nuclear receptor involved in adipocyte differentiation, promotes secretion of anti-hyperglycemic adipokines
mutations cause diabetes
Thiazolidinediones for T2DM
PPARy agonist
promotes secretion of anti-hyperglycaemic adipokines -> increases lipolysis
sensitize cells to insulin
collectively reduces insulin resistance in liver/perpheral tissues
Metformin for T2DM
supresses glucose release from liver
activates AMPK
increase lipolysis in muscles and liver -> improve insulin receptor signalling
suppress glucose release from liver
Sulphonylureas for T2DM
bind sulphonylurea receptors on beta cell membranes
block ATP sensitive K+ channels on b cells -> K+ accumulates insise -> cell depolarises -> Ca2+ channels open -> allow insulin secretion y exocytosis
macro complications of T2DM
hyperglycemia -> macrovascular disease -> damage to medium/large blood vessels -> coronary heart disease, cerebrovascular disease, peripheral vascular disease
micro complications of T2DM
hyperglycemia -> microvascular disease -> damage to small blood vessels -> retinopathy, nephropathy, neuropathy
ROS generation
from excess glucose and FFA
-> micro- and macro-vascular complications of T2DM
AGE generation
from excess sugar molecules and proteins
-> vessel damage
AGE effect on blood vessels
AGE crosslinks with collagen
-> basal membrane of endothelium thickens
-> traps LDL and IgGs
-> oxidation, complement activation, inflammation
-> vessel damage
