Insulin Flashcards
What is the main action of insulin?
Enhances uptake, utilisation and storage of glucose
Describe an insulin receptor (IR)
Its one large transmembrane glycoprotein complex
With two alpha and two beta subunits linked by disulphide bridges
Describe the alpha subunit in the insulin receptor (IR)
Extracellular
Insulin binding domain
Describe the beta subunit in the insulin receptor (IR)
Transmembrane proteins with intrinsic tyrosine kinase activity
Which methods can be used to determine IR expression in CNS?
Immunocytochemistry
In situ hybridisation
Western blots & PCR
Outline in situ hybridisation
Labelled anti-sense oligonucleotide probes inserted which are specific to IR subunits
Detects mRNA
How does peripheral insulin get into the brain?
Active transport across the BBB via a regulated, saturable insulin transporter
How may insulin be released locally from the CNS?
Depolarisation of insulin mRNA expressing neurons should cause insulin relase
What is the main role of insulin in the CNS?
Mediate cerebral glucose uptake in glial cells
Hyper-insulinaemia is a risk factor for Parkinson’s disease
True/False
False
risk factor for dementia
How does insulin administration affect memory formation?
Improves it
Craft et al 1999
Why doe we think that insulin is relevant to learning and memory?
IRs concentrated in brain regions involved in learning and memory
Hippocampus and amygdala
How does insulin affect synaptic strength?
Increases synaptic strength of excitatory synapses
Man et al 2000
What is streptozotocin?
A diabetic inducing agent
What effect does streptozotocin have on memory?
Memory impairment
Which NMDAR features does insulin enhance?
NMDAR single channel currents
NMDAR mediated synaptic transmission
Which isoforms of NMDAR does insulin enhance?
NR1-NR2A and NR1-NR2B
These are prominent in the hippocampus!
Outline insulin receptor signal transduction
Binds to alpha subunit
Causes autophosphorylation of beta subunits
Stimulates intrinsic tyrosine kinase activity
Phosphorylation then triggers the activation of other signalling cascades
Phosphorylation of IR causes activation of which signalling cascades?
IRS proteins and Src proteins (PI3K)
Shc protein
What does Shc protein do?
Mediates association of IR with Grb-2-SOS complex
Activates Ras-Raf-MAPK
Which inhibitors block insulin action?
IR tyrosine kinase inhibitors eg tyrphostin (Skeberdis 2001)
PKC inhibitors (Liao 1999)
Src TK inhibitors (Liao and Leonard 1999)
How does insulin effect NMDAR surface expression?
Increases exocytosis
Inhibits endocytosis
How does insulin increase NMDAR expression and how do we know?
Via SNAP-25 dependent exocytosis
Botox injections cleave SNAP-25 and prevents this exocytosis
With botox there is reduced NDMAR current
Skeberdis 2001
How does SNAP-25 driven insulin receptor exocytosis work?
IR activated Activates IRS proteins Activates PI3K Then activates abnormal protein kinase C This increases SNAP-25 activity which then causes the receptors exocytosis
Through which signalling pathways does insulin increase NMDA responses?
PI3K and MAPK
Because application of inhibitors of these stop the increase in current
How may insulin modulate LTD?
Depressing AMPAR mediated EPSCs in hippocampal neurons by reducing surface AMPAR expression
Man et all 2000
Low concentrations of insulin induce LTP
True/false
False
Low conc produces transient synaptic depression
High conc induces LTD
Insulin decreases NMDAR EPSCs
T/F
F
Depresses AMPAR EPSCs but not NMDARs
Which AMPAR subunit does insulin supress?
GluA2
But not GluA1
How do we know insulin affects AMPAR subunits?
Immunocytochemistry shows less of them
Man et all 2000
How does hypertonic sucrose affect insulin mediated subunit endocytosis?
Inhibits clathrin coating the vesicle which means it cant endocytose
What can be given to block insulin-mediated endocytosis of AMPAR?
Hypertonic sucrose
Post-synaptic dialysis with Anti-IR Abs
Why do we think insulin and memory are linked?
In rodents, MWM training upregulated IR mRNA and protein in CA1 region
Increase in IR staining also observed
Zhao et al 1999
What biochemical changes were observed following training in the MWM?
Increased tyrosine phosphorylation in 52 and 66 kDa proteins
Increase in Shc-66
Accumulation of Grb (52kDa)
Increase in active MAPK
Zhao et al 1999
Which neurodegenerative disorder is insulin linked to?
AD
How may insulin be useful in AD treatment?
Prevents Ab derived diffusible ligands (ADDL) induced synapse loss
Which signalling pathway does insulin enhance NMDA via?
PI3K-MAPK
when this is blocked u dont get insulin mediated increase in NMDARs
What is insulin induced LTD dependent on?
Postsynaptic IR activation
