Post on 22-Sep-2020
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)