Cell Communication

The Cellular “Internet”

Multicellular organisms:

cells must communicate with one another

to coordinate their activities

Unicellular oganisms:

communication also important

Signal transduction pathway: a series of steps

signal on a cell‟s surface specific cellular response

Similar in all organisms

Direct contactPlasmodesmata in plant cells

Gap junctions in animal cells

Local (Short-Distance) Signaling

Local (Short-Distance) Signaling Direct contactPlasmodesmata in plant cells

Gap junctions in animal cells

What membrane associated molecule

plays a role in cell-cell recognition?

Cell-cell recognitionMembrane-bound surface molecules can interact & communicate

Signals can pass between adjacent cells through junctions

Local (Short-Distance) SignalingMessenger molecules can be secreted by the signaling cell

Paracrine signaling:One cell secretes (releases) molecules that act on nearby “target”

cells

Ex: growth factors (stimulate nearby cells to grow & multiply)

Synaptic Signaling:Nerve cells release chemical messengers (neurotransmitters) that

stimulate the target cell

Long-Distance Signaling

Endocrine (hormone) signalingSpecialized cells release hormone

molecules

hormones travel to target cells

elsewhere in the organism

Ex. Insulin

Ex. Ethylene

How do hormones get into blood vessel?

diffusion

How do hormones get to target?

circulatory system

The Three Stages of Cell Signaling The “receiving end” of a cellular conversation:

1. Reception2. Transduction3. Response

Stage 1: Reception Target cell “detects” a signal molecule coming from

outside the cell The signal is detected when it binds to a protein on the cell‟s

surface or inside the cell

The signal molecule “searches out” specific receptor proteins The signal molecule is a ligand

• It is a molecule that specifically binds to another one (think enzymes!)

http://www.youtube.com/watch?v=bU4955rLv_8&feature=player_embedded

http://www.youtube.com/watch?v=2bbBrpgeheY&feature=related

Stage 2: Transduction

The signal is converted into a form that can bring about a specific cellular responseOne signal-activated receptor activates another

protein, which activates another molecule, etc.

These act as relay molecules

Often the message is transferred using protein kinases

protein kinases: transfer phosphate groups from ATP molecules to proteins

http://www.youtube.com/watch?v=3jMBNesc-8k (dom)

Stage 2: Transduction

http://www.youtube.com/watch?v=MNVB7K-MDws

http://www.youtube.com/watch?NR=1&v=NaOBRvAFiJQ

Stage 3: Response

The signal that was

passed through the

signal transduction

pathway triggers a

specific cellular

responseExamples: enzyme action,

cytoskeleton rearrangement,

activation of genes, etc., etc.

Diagram example:

transcription of mRNA

Regulation by chemical messengers

axon

endocrine gland

receptor proteins

target cell

Neurotransmitters released by neurons

Hormones release by endocrine glands

receptor proteins

hormone carried by blood

neurotransmitter

Lock & Keysystem

Action of protein hormones

activatesenzyme

activatesenzyme

activates enzyme

ATP

produces an action

P

1

2

3

cytoplasm

receptorprotein

response

signal

secondarymessengersystem

signal-transduction pathway

acts as 2nd messenger

target cell

plasma membrane

binds to receptor protein

proteinhormone

ATPactivatescytoplasmicsignal

cAMP

GTP

activatesG-protein

transduction

nucleus

target cell

DNAmRNA

protein

blood

proteincarrier

S

S

S

S

Action of lipid (steroid) hormones

binds to receptor protein

cytoplasm

becomes transcription factor

ex: secreted protein = growth factor (hair, bone, muscle, gametes)

2

4

6

cross cell membrane

1

steroid hormone

mRNA read by ribosome5

plasma membrane

protein secreted7

3

adrenal gland

Ex. Action of epinephrine (adrenaline)

activatesprotein kinase-A

activatesglycogen phosphorylase

activatesadenylyl cyclase

epinephrine

liver cell

releasedto blood

1

25

receptorproteinin cell membrane

cytoplasm

6glycogen

activatesphosphorylase kinase

GTP

cAMP

4

activatesG protein

ATP

glucose

activates GTP

3

signal

transduction

response7

GDP

Benefits of a 2nd messenger system

Amplification!

signal

receptor proteinActivated adenylyl cyclase

amplification

amplification

amplification

amplification

GTP G protein

product

enzyme

protein kinase

cAMP

Not yetactivated

1

2

4

35

6

7

FAST response!

amplification

Cascade multiplier!

The Specificity of Cell Signaling

The particular proteins that a cell possesses determine which signal molecules it will respond to and how it will respond to them

Liver cells and heart cells, for example, do not respond in the same way to epinephrine because they have different collections of proteins

G Protein-Linked

Transmembrane protein receptors that

interact (can bind) with a G protein

capable of binding GDP and GTP

An activated (GTP bound) G protein

subunit (there are 3 subunits per G

protein) separates & „seeks‟ an protein

that can then create a cascade of effects

in the cell

G Protein-Linked

But Wait!

The same signal can have different effects in different tissuesG protein-linked receptor

can activate an effector protein in one tissue type, but inhibit in another

Can You…

Explain why cells need to signal/ communicate? How (if) multicellular organisms differ from single-celled?

Describe the 3 major types of signals? The 3 steps in signal transduction?

Identify the 2 locations of receptors? The 3 major types of plasma membrane receptors (AND how they work)?

Second Messenger:

Released into cytoplasm, where they

may have numerous effects (ie.

interact with multiple target proteins)

NOT - typically ions or small, water-

soluble molecules (like cAMP)

Act as cofactors or allosteric

regulators

Amplify signals

Second Messenger:

Protein Kinase Cascades

„Domino effect‟ that results in signal

Why? At each step, modify an inactive protein kinase into an active one; each can catalyze many phosphorylations of target proteinsAgain, by producing different target proteins in

different tissues, can have varying responses to the same signals

So, In Summary:

Signals are cues from the organism or environment & can be autocrine, paracrine, or hormone

Signal-transduction pathways consist of: a receptor, transduction, and an effect (response)

Receptors can be on the membrane (ion channels, protein kinases, G protein-linked) OR in the cytoplasm

Transduction can be direct (on membrane) or indirect(utilizing second messengers); these and/or transduction cascades can amplify signals

Effects vary depending upon signal but often result in changes in transcription or enzyme activity

Cells can communicate directly via gap junctions (animal) or desmosomes (plant)