Receptor Mediated Endocytosis

Front 1

Molecules that enter cell via receptor mediated endocytosis

Back 1

-vesicle formed from membrane and clathrin-coated pit to form early endosome

toxins, antibodies, viruses, hormones, GF, serum transport proteins, LDL

Front 2

Receptor-mediated endocytosis
Step 1

Back 2

Receptor-ligand binding
-receptor is transmembrane protein, moves laterally in plane of membrane, binding sites for ligand on extracellular domain
-ligand is extracellular, binds to receptor, causes rapid activation of second messengers in a cascade that affects the cell
*Endocytosis not always required to affect (stimulate/inhibit) the cell. Just the ligand binding to the receptor can cause the signaling of the receptor mediated cascade

Front 3

Receptor-mediated endocytosis
Step 2
importance of concentration
affect of temperature

Back 3

concentration of ligand/receptor in patches or at one pole of the cell

-concentration allows regulated entry of fluids, w/o this organization there would be too much fluid entry making more work for the cell

-concentration mediated by arrestins (on membrane), signaling peptides (on receptor), and adaptins (clathrin coat)

-temperature: body temp promotes patching and internalization

Front 4

Receptor-mediated endocytosis
Step 3
affect of temperature

Back 4

Receptor-ligand moves to clathrin-coated pits
-pits collect receptors and concentrate them
-multiple receptors can be taken in to one clathrin-coated pit

Temperature: body temp promotes internalization; low temperature may prevent entry of ligand, but not binding to the receptor

Front 5

Formation of clathrin coated pits

Back 5

Clathrin coat subunits collect on membrane and assemble to form vesicle formation.
-vesicle pinches off to form vesicle
-coat disassembly to form uncoated vesicle

-they appear and disappear in same place: constantly being reformed, and somehow organized, possibly held in place by cytoskeletal system

Front 6

How to receptors know to enter clathrin-coated pits?

Back 6

-B-arrestins: proteins that guide receptor to pit, facilitates binding b/t signal sequence on receptor and adapter proteins of clathrin coat

-Receptor signal sequence on C-terminus cytoplasmic end (Tyr-X-Arg-Phe) and binds to adaptin molecules (AP-2) in clathrin coat, can stimulate more clathrin to accumulate

Front 7

Pit to vesicle

Back 7

-pit invaginates and connected to PM by a stem
-Dynamin (GTPase) binds to stem
-GTP hyrolysis= energy to pinch off stem to form vesicle with receptor-ligand cargo
-coat disassembled then fuses with other vesicles according to specific sorting signal to form the early endosome

Front 8

Function of LDL receptors
-defect in LDL receptors

Back 8

-carry cholesterol into cell via receptor mediated endocytosis
-LDL receptor then bound by adaptin in clathrin coated pits= concentrates receptors and encourages vesicle formation

Defect: no coat-binding site on LDL receptor, prevents them from binding to Adaptin-2; they do not enter pits and cannot be brought into the cell
Result: high serum cholesterol, b/c LDL provides critical mech. for reducing LDL levels by bringing it into the cell
*hypercholesterolemia: familial defect, LDL binds cholesterol but cannot bring it into cells; causes early heart attacks and artherosclerosis

Front 9

Formation of early endosome to late endosome

Back 9

-loss of coat and fuse (pH 5.9-6.0)
-fusion via GDP-bound rab 5 sorting signal
-at low pH, some receptors are released from the ligand and recycled to PM= late endosome formed
-receptor recycling compartment has higher pH of 6.4-6.5
-Not all receptors are recycled
some are: like transferrin
some are not, and are degraded: like EGF receptor

Front 10

Late endosome characteristics and function
clinical correlation

Back 10

-pH lowers to 5.0-6.0
-Rab sorting signal changes to rab7-GDP: allows for fusion and communication with trans golgi network
-membrane rich
-Distinguished by LBPA, a lipid

-late endosome: enzymes break down ligand, proteins, lipids, and receptors; hydrolytic enzymes release cholesterol so it can be used in the cell

-communicates with golgi complex: vessicles from trans golgi deliver ACID HYDROLASES (sorted in golgi, bound to mannose-6-P receptors)
-addition of acid hydrolases forms late endosome and eventually a lysosome

-Recycles to golgi:
receptors that are not degraded
mannose 6P receptor back to transgolgi network
-eventually fuses with lysosome

-LAMP+ and M6PR+

-CC: site of problem in Neiman Pick Disease type C: cannot progress past late endosome

Front 11

Fusion of late endosome with lysosome
clinical correlation

Back 11

pH falls from 4-5
endpoint of endocytotic pathway
-receptors degraded
-LAMP (lysosomal associated membrane positive) complex bodies
-Not mannose 6P receptor positive (if receptors recycled to trans golgi)

LAMP+ and M6PR-

-site of problem in Neiman Pick Disease Types A and B

Front 12

Neiman Pick Disease

Back 12

Autosomal recessive disorder

Types A and B:
-deficiency in acid sphingomylienase (ASM)
-lack of ASM in lysosome causes a lipid/membrane buildup; eventually kills cells and damages organs (spleen and liver)
-seen in macrophages (lots of lysosomes)

Type A: neurological tissues, death w/in 2-3 yrs
Type B: enlarged spleen, respiratory problems, cardiovascular problems, can live into adulthood

Type C (NPC):
-problem with late endosome
-mutated NPC1 protein- needed for cholesterol transport
-cholesterol accumulates, late endosomes are stuffed and stuck in traffic, they cannot progress past endosome stage
-also blocks transport of M6PR back to golgi (late endosomes are LAMP+ and M6PR+)
-always fatal, usually affects school age children
-symptoms: jandice at birth, enlarged spleen and/or liver, difficulty with up and down eye movements (vertical supranuclear gaze palsy); slurred irregular speach (dysarthria); learning difficulties and progressive intellectual decline (dementia); sudden loss of muscle tone that may lead to falls (cataplexy)