Bio 111 Cell Communication

why do cells need to communicate

-sense information from the environment

-cells interacting/coordinating for a task

-changes in transcription

-changes in enzyme activity

-uptake molecules from the environment

four essential elements in cell communication

1) signaling cell

2) ligands/signaling molecules

3) receptor cell

4) target/responding cell

signaling cell

distributes information

ligands

message

receptor cell

binds to specific ligands and receives message

target/responding cell

receives message and does something with it

types of signaling

1) endocrine

2) paracrine

3) autocrine

4) juxtacrine

how are signal types classified

by distance and ligand

which signaling types require the signaling cell to secrete an extracellular ligand

endocrine, paracrine, autocrine

which signaling types require the ligand to diffuse a short distance

paracrine and autocrine

which signaling types require the ligand to travel in the bloodstream

endocrine

which signaling types require the ligand to travel in the bloodstream

endocrine

which signaling types use a membrane attached ligand

juxtacrine

in which signaling types is the responding cell the signaling cell

autocrine

endocrine signaling

the signals travel long distances through blood or lymph to reach a responding cell

target cells in a different organ, so are released by glands

think of endocronologist (work with pancreas gland)

paracrine signaling

two cells are close so signals travel short distances

for example, they would be in the same tissue

autocrine signaling

signaling and responding cell are the same

talking to itself

juxtacrine signaling

ligand must be transmembrane on surface of signaling cell

contact dependent signaling, so cells must be touching

useful for forming borders between two tissue types

steps to cell signaling

1) receptor activation

2) signal transduction

3) response

4) termination

receptor activation

ligand is bound by receptor and receptor is activated

signal transduction

receptor is at cell’s surface and we need a way for info to get into cell so we transmit info into the cell and then amplify the signal to get a robust respnse

amplificator

don’t want to activate one single enzyme molecule, we need to increase response at multiple levels to get a big answer in the cell

response

cell does something

depends on the cell and the signal (ex: activate an enzyme, change whether or not a gene is on/off, cell division, et cetera

termination

cell stops responding

bc of cell needs and environmental changes so it is prepared to receive new info and doesn’t excessively respond

how receptors are turned on/off

the receptor recognizes the ligand by its R group, and the ligand binds to the binding domain which causes the receptor to change shape and become a receptor ligand complex, the conformational change results in a change in function

types of surface receptors

1) g protein coupled receptor

2) receptor kinase

g protein signaling

uses GTP/GDP binding G proteins

G protein binding

if GTP bound it is active, if it is GDP bound it is inactive

G protein coupled receptor

1) the receptor is off because the ligand has not binded to it yet

2) because the receptor is off, the G protein is also inactive

3) the ligand binds and the receptor is activated

4) the G protein interacts with the activated receptor and it exchanges the GDP for GTP to activate

5) will transmit information to kick off signal transduction

6) one of its targets is adenylyl cyclase

adenylyl cyclase

common target of G protein activation, takes ATP molecules and converts it into cyclic AMP

cAMP

second messenger of cell since it activates targets inside the cell, which allows for amplification

amplification 

signal transduction gets a bigger response, turns switches on at every level

termination

need to reverse what happened (turn everything off) so everything isn’t used up/continues excessively

1) ligand leaves receptor → receptor is inactive again

2) GTP hydrolyzes into GDP in G protein to turn itself off

3) additional enzymes inactivate targets so everything is “off”

receptor kinases

enzymes that add phosphate groups to other molecules

1) receptor kinase has phosphorylation sites where ligand binds, and its cytoplasmic domain is also a receptor of kinase

2) receptor kinase always works in pairs

3) ligand binds and causes receptor kinase to dimerize

4) the kinase receptors phosphorylate each other and activate the receptors

5) once they are phosphorylated the cellular signal transduction proteins bind and the proteins are thus activated

Ras and MAPK pathway

*works similar to G proteins → if it is GTP bound it is active, if it is GDP bound it is inactive

1) Ras holds GDP and is activated by the kinase receptor when the signaling molecule binds to the inactive receptor

2) now Ras is holding GTP and stars a chain reaction of kinases continuously phosphorylating and activating other kinases until the target enters the nucleus and turns genes “on/off”

3) another opportunity for amplification and by activating targets causes cell division

4) cell division needs to be terminated (cancer)

5) termination → ligand leaves, GTP hydrolyzes to GDP, phosphatase remove phosphates and kinases/targets become inactive