Transcript AP BIO Chp 11 Cell to Cell Communication

Cell Communication
Overview: The Cellular
Internet
 Cell-to-cell
communication is
absolutely essential for multicellular
organisms
 Nerve cells must communicate pain
signals to muscle cells (stimulus) in
order for muscle cells to initiate a
response to pain
 Biologists
have discovered some
universal mechanisms of cellular
regulation
External Signals
Signal Transduction Pathway

Yeast cells identify their mates by cell
signaling (early evidence of signaling)
1
2
3
Exchange of
mating factors.
Each cell type
secretes a
mating factor
that binds to
receptors on
the other cell
type.
Mating. Binding
of the factors to
receptors
induces changes
in the cells that
lead to their
fusion.
New a/ cell.
The nucleus of
the fused cell
includes all the
genes from the
a and a cells.
 factor
Receptor
a

Yeast cell,
mating type a
 factor
Yeast cell,
mating type 

a
a/
Signal Transduction Pathways
 Convert signals on a cell’s
surface into cellular responses
 Are similar in microbes and
mammals suggesting an early,
and common, origin.
Local and Long-Distance
Signaling
•
Cells in a multicellular organism (tissues,
organs, systems) communicate via chemical
messengers
•
Sometimes cells need to send messages to
itself (autocrine signaling).
•
Messages need to be sent both shorter
distances (paracrine signaling), as well as longer
distances (endocrine signaling).

Animal and plant cells
 Have cell junctions that directly connect
the cytoplasm of adjacent cells
Plasma membranes
Gap junctions
between animal cells
Plasmodesmata
between plant cells
Figure 11.3 (a) Cell junctions. Both animals and plants have cell junctions that allow molecules
to pass readily between adjacent cells without crossing plasma membranes.
 In
local signaling, animal cells
 May communicate via direct contact
Figure 11.3(b) Cell-cell recognition. Two cells in an animal may communicate by interaction
between molecules protruding from their surfaces.
 In
other cases, animal cells
communicate using local regulators
Local signaling
Target cell
Electrical signal
along nerve cell
triggers release of
neurotransmitter
Neurotransmitter
diffuses across
synapse
Secretory
vesicle
Local regulator
diffuses through
extracellular fluid
(a) Paracrine signaling. A secreting cell acts
on nearby target cells by discharging
molecules of a local regulator (a growth
factor, for example) into the extracellular
fluid.
Target cell
is stimulated
(b) Synaptic signaling. A nerve cell
releases neurotransmitter molecules
into a synapse, stimulating the
target cell.
Long-Distance Signaling
Endocrine signaling
• A hormone is a chemical released by a cell in one
part of the body, that sends out messages that
affect cells in other parts of the organism
• All multicellular organisms produce hormones
plant hormones are also called phytohormones
• Hormones in animals are often transported in the
blood
11

In long-distance signaling
 Both plants and animals use hormones
(e.g. in animals-insulin, in plants-auxin)
Long-distance signaling
Endocrine cell
Blood
vessel
Hormone travels
in bloodstream
to target cells
Target
cell
Figure 11.4
(c) Hormonal signaling. Specialized
endocrine cells secrete hormones
into body fluids, often the blood.
Hormones may reach virtually all
C body cells.
The Three Stages of Cell
Signaling

Earl W. Sutherland
 Discovered how the hormone epinephrine acts on
cells, stimulating the breakdown of glycogen in the
liver and skeletal muscle cells
Sutherland’s Three Steps
 Sutherland
suggested cells that
are receiving signals went through
three processes:
Reception
Transduction
Response
 Overview
EXTRACELLULAR
FLUID
1 Reception
of basic cell signaling
Plasma membrane
CYTOPLASM
2 Transduction
3 Response
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway
Signal
molecule
Figure 11.5
Step One - Reception
 Reception
occurs when a signal
molecule binds to a receptor protein,
causing it to change shape
 Receptor proteins are on the cell’s
surface.
 The
binding between signal molecule
(ligand) and receptor is highly specific
 A conformational change in a receptor
is often the initial transduction of the
signal.
Step Two - Transduction



The binding of the signal molecule alters the
receptor protein in some way
The signal usually starts a signaling cascade, a
series of enzyme catalyzed reactions, known as a
signal transduction pathway
Multi-step pathways can amplify a signal
Step Three - Response
 Cell
signaling leads to regulation of
cytoplasmic activities or initiation of
transcription
 Signaling
pathways regulate a variety
of cellular activities
Example of Pathway

Steroid hormones bind to intracellular receptors
Hormone
EXTRACELLULAR
(testosterone)
FLUID
Receptor
protein
Plasma
membrane
Hormonereceptor
complex
1 The steroid
hormone testosterone
passes through the
plasma membrane.
2 Testosterone binds
to a receptor protein
in the cytoplasm,
activating it.
3 The hormone-
DNA
mRNA
NUCLEUS
Figure 11.6
CYTOPLASM
receptor complex
enters the nucleus
and binds to specific
genes.
4
New protein
The bound protein
stimulates the
transcription of
the gene into mRNA.
5 The mRNA is
translated into a
specific protein.
 Other
pathways regulate genes by
activating transcription factors that
turn genes on or off
Growth factor
Receptor
Phosphorylation
cascade
Reception
Transduction
CYTOPLASM
Inactive
transcription Active
transcription
factor
factor
P
Response
Figure 11.14
DNA
Gene
NUCLEUS
mRNA
Termination of the Signal
 Signal
response is terminated
quickly by the reversal of ligand
binding
Receptors in the Plasma
Membrane
 There
are three main types of
membrane receptors:
 G-protein-linked
 Tyrosine kinases
 Ion channel
 G-Protein-Coupled
Signal-binding site
•
•
•
•
Segment that
interacts with
G proteins
•
G-protein-linked
Receptor
CYTOPLASM
GPCR receives signal
GPCR activates G-protein by exchanging a GTP for
a GDP
G protein binds to an effector protein
Figure 11.7
Effector protein initiates
response – production of
nd
2++
2 messengers [Ca
, cAMP, for example]
GPCR signaling is deactivated when GTP is
hydrolyzed GTP GDP + Pi
Plasma Membrane
GDP
Receptors
G-protein
(inactive)
Enzyme
Activated
Receptor
GDP
Signal molecule
GTP
Activated
enzyme
GTP
GDP
Pi
Cellular response
Inactivate
enzyme
Some examples of systems using
G-linked receptors include:
• yeast mating,
• epinephrine,
• neurotransmitters
“different proteins are receptors
for different neurotransmitters…”
Acetylcholine is commonly secreted
at neuromuscular junctions,
the gaps between neurons and muscle cells,
where it stimulates muscles to contract.
At OTHER junctions, it may produce
an inhibitory post-synaptic response.
25
Enzymes, like cholinesterase
degrade neurotransmitters.
The degraded neurotransmitters
are then rapidly recycled,
and taken up by the pre-synaptic cell.
26
 Receptor
tyrosine kinases
Signal-binding site
Signal
molecule
Signal
molecule
Helix in the
Membrane
Tyr
Tyrosines
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Receptor tyrosine
kinase proteins
(inactive monomers)
CYTOPLASM
Tyr
Dimer
Figure 11.7
Activated
relay proteins
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
6
ATP
Activated tyrosinekinase regions
(unphosphorylated
dimer)
6 ADP
P Tyr
P Tyr
P Tyr
Tyr P
Tyr P
Tyr P
Fully activated receptor
tyrosine-kinase
(phosphorylated
dimer)
P Tyr
P Tyr
P Tyr
Tyr P
Tyr P
Tyr P
Inactive
relay proteins
Cellular
response 1
Cellular
response 2
Some examples of signaling pathways that employ
tyrosine kinase receptors include:
• transformation of glucose to glycogen
• glucose export, and
• activation of transcription factors promoting
cell growth and differentiation
Ability of tyrosine kinase receptors to amplify
the original signal is an important difference
between
tyrosine kinase and G-coupled proteins
28
 Ion
channel receptors
Signal
molecule
(ligand)
Gate closed
Ligand-gated
ion channel receptor
Ions
Plasma
Membrane
Gate open
Cellular
response
Gate close
Figure 11.7
Some examples of signaling pathways that employ
gated ion channel receptors include:
• Nerve impulses
• Muscle contraction
[due to sodium &/or potassium gates
opening or closing]