Regulated Exocytosis

Define exocytosis

The process of vesicle fusion with the plasma membrane

Define secretion

The release of soluble vesicular content from the cell following exocytosis

constitutive exocytosis

Constant process of vesicles budding from Golgi and fusing directly with the membrane. Found in all cell types

purpose of constitutive exocytosis

continual transport of newly synthesised lipids/proteins to membrane, and secretion of molecules eg ECM components

regulated exocytosis

Vesicles retained until signal received eg calcium increase. Signal promotes vesicle fusion

Describe the process of insulin secretion via regulated exocytosis due to calcium ion release

• glucose uptake and metabolism increases → ATP:ADP increases

• ATP sensitive K+ channels close → depolarisation

• voltage gated calcium channels open

• calcium ion influx → insulin granule exocytosis

glucose phosphorylation → glucose-6-phosphate

Commits glucose to further metabolism. Catalysed by hexokinase and glucokinase

hexokinase

Ubiquitously expressed enzyme with high glucose affinity ensuring sufficient glucose metabolism even when BGC low

glucokinase

Activity varies within physiological BGC range as it has lower affinity for glucose than hexokinase and greater Vmax. mainly found in beta cells

first phase insulin secretion

released from readily releasable granules docked at membrane

second phase insulin secretion

requires recruitment of cytosolic reserve pool

synaptobrevin

vSNARE which complexes with membrane SNAREs eg syntaxin and SNAP23 → vesicle fusion

synaptotagmin

Found on insulin granules. When calcium bound, interacts with SNARE protein and membrane phospholipids to increase chance of membrane fusion

Insulin effects on adipocytes and skeletal muscle

stimulates exocytosis of GLUT4 vesicles

What happens as a result of insulin receptor autophosphorylation?

• phosphorylated residues bind IRS and phosphorylate 4 tyrosines

• PI3K binds, converts PIP2 → PIP3

• PIP3 recruits PDK1 kinase

• PDK1 activated, phosphorylates kinases eg PKB

• PKB phosphorylates GLUT4 vesicle proteins

What SNARE protein interactions are required for GLUT4 vesicle exocytosis?

vesicular VAMP with membrane syntaxin and SNAP23

AS160

retains GLUT4 vesicles. Activated PKB phosphorylates AS160 to inactivate it

Why does hyperglycaemia occur?

Loss of:

• insulin stimulated glucose uptake

• repression of gluconeogenesis and glycogen breakdown

characterisation of type 2 diabetes

high BGC due to:

• insulin resistant target tissue

• insufficient production/secretion of insulin

pancreatic dysfunction due to type II diabetes

pancreas sometimes responds to insulin resistance by producing more insulin, causing stress

Why does type II diabetes/insulin resistance arise?

Strong link with obesity but unsure of how this causes resistance. Many different mechanisms of disrupting insulin signalling implicated

metformin

inhibits gluconeogenesis in liver - used to treat type II diabetes

sulfonylureas

bind and close ATP gated K+ channels triggering insulin secretion

sequential compound exocytosis

primary fused vesicles become targets for secondary fusion events with vesicles deeper within the cell

multigranular compound exocytosis

secretory vesicles fuse with each other in the cell before fusing with the membrane

mast cell degranulation

mediators stored in large metachromatic granules released by regulated exocytosis. Signal = IgE crosslinking receptor