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Cell Communication

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Cell Communication For cells to function in a biological system, they must communicate with other cells and respond to their external environment. – PowerPoint PPT presentation

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Title: Cell Communication


1
Cell Communication
For cells to function in a biological system,
they must communicate with other cells and
respond to their external environment.
2
Enduring understanding Cells communicate by
generating, transmitting and receiving chemical
signals.
  • Essential knowledge
  • Cell communication processes share common
    features that reflect a shared evolutionary
    history.
  • Cells communicate with each other through direct
    contact with other cells or from a distance via
    chemical signaling.
  • Signal transduction pathways link signal
    reception with cellular response.
  • Changes in signal transduction pathways can alter
    cellular response.
  • Organisms respond to changes in their external
    environments.

3
Chemical Signals.
  • Can direct complex processes, ranging from cell
    and organ differentiation to whole organism
    physiological responses and behaviors.
  • Can allow cells to communicate without physical
    contact.
  • The distance between the signal generating
    cell(s) and the responding cell can be small or
    large.

4
Methods of cell communication
  • Three general methods of cell communication
  • Diffusible chemical signals (messengers) that
    travel through the organism from one location to
    another (Close or long distances)
  • Physical contact between adjacent cell plasma
    membranes
  • Direct cytoplasmic contact via gap junctions

5
CELL COMMUNICATION
Part 1 An Overview of Cell Signaling
1.Cell signaling evolved early in the history of
life 2.Communicating cells may be close together
or far apart 3.The three stages of cell signaling
are reception, transduction, and response
6
Introduction
  • Cell-to-cell communication is absolutely
    essential for multicellular organisms.
  • coordinate the activities within individual cells
    that support the function of the organism as a
    whole.
  • Use of pheromones to trigger reproduction and
    developmental pathways
  • Important for many unicellular organisms.
  • finding a mate
  • population density (quorum sensing)
  • Response to external signals by bacteria that
    influences cell
  • allowed some organisms to evolve without having a
    nervous system.

7
1-Cell signaling evolved early in the history of
life
  • One topic of cell conversation is mating.
  • Ex The yeast Saccharomyces cerevisiae, the yeast
    of bread, wine, and beer, identifies its mates by
    chemical signaling.
  • There are two sexes, a and alpha, each of which
    secretes a specific signaling molecule, a factor
    and alpha factor respectively.
  • These factors each bind to receptor proteins on
    the other mating type.

8
  • Cell signaling has remained important in the
    microbial world.
  • Myxobacteria, soil-dwelling bacteria, use
    chemical signals to communicate nutrient
    availability.
  • When food is scarce, cells secrete a signal to
    other cells leading them to aggregate and form
    thick-walled spores.

9
2-Communicating cells may be close together
(LOCAL) or far apart
10
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11
Local signaling
Example of Localized signaling
  • c. In synaptic signaling, a nerve cell produces
    the neurotransmitter that diffuses to a single
    cell that is almost touching the sender.
  • An electrical signal passing along the nerve cell
    triggers secretion of the neurotransmitter into
    the synapse.

12
Local signaling d. Contact dependent
Touching
  • Gap Junctions
  • Narrow tunnels between animal cells that consist
    of proteins called connexons.
  • Allows the movement of ions and small molecules
    from cell to cell.
  • Plasmodesmata
  • Narrow channels between plant cells
  • A narrow tube of Endoplasmic reticulum.
  • Material exchange

13
Local signaling d. Contact dependent
  • Signaling substances dissolved in the cytosol
    pass freely between adjacent cells.

Touching
Clip Start at 530
14
Long-Distance Signaling
  • Endocrine (hormone) signaling
  • Specialized cells release hormone molecules,
    which travel (usually by diffusion through cells
    or through the circulatory system) to target
    cells elsewhere in the organism

Hormone Control Clip
15
Long-Distance Signaling
  • Plants also use hormones to signal at long
    distances.

-In plants, hormones may travel in vessels, but
more often travel from cell to cell (auxin) or by
diffusion in air (Ethylene). -Ethylene gas
in fruit ripening
16
3. The three stages of cell signaling reception,
transduction, response
McGraw-Hill Dehydration Response Example Clip
EK Signal transduction pathways link signal
reception with cellular response.
17
The three stages of cell signaling
CELL COMMUNICATION
Part 2 Signal Reception and the Initiation of
Transduction
Step 1 In reception, a chemical signal binds to
a cellular protein, typically at the cells
surface.
A signal molecule (ligand), binds to a receptor
protein, causing the protein to change shape
18
Step 1. A signal molecule binds to a receptor
protein causing the protein to change shape
  • Receptor protein recognizes the signal molecule
    (ligand).
  • Recognition receptor on target cell is
    complementary in shape.
  • The ligand attaches to the receptor
    protein, the receptor typically undergoes
    a change in shape.
  • 1. This may activate the receptor so that
  • it can interact w/other molecules inside
    the cell.
  • 2. Can lead to the aggregation of
  • receptors.
  • Signal molecule does not enter
  • cell.

19
Most signal receptors are plasma membrane proteins
  • Most signal molecules are water-soluble and too
    large to pass through the plasma membrane.
  • Three major types of receptors
  • G-protein-linked receptors
  • short for guanine nucleotide binding proteins
  • Tyrosine-kinase receptors
  • Ion-channel receptors

20
  • PROJECTS START HERE

21
G protein-linked receptors
  • 1. G-protein-linked receptors consists of a
    receptor protein associated with a G-protein on
    the cytoplasmic side.
  • The receptor consists of seven alpha helices
    spanning the membrane.
  • Effective signal molecules include yeast mating
    factors, epinephrine, other hormones, and
    neurotransmitters.

22
G protein-coupled receptors
G protein-linked receptors
  • An example of a G protein-linked receptor is the
    epinephrine receptor.
  • Epinephrine stimulates glycogen breakdown
  • G-protein-linked receptors and G-proteins mediate
    a host of critical metabolic and developmental
    processes (e.g., blood vessel growth and
    development).

23
G protein-linked receptors
  • Uses the exchange of Guanosine diphosphate (GDP)
    for Guanosine triphosphate (GTP) as a molecular
    "switch" to allow or inhibit biochemical
    reactions inside the cell

24
G protein-coupled receptors
  • Large family all with 7 membrane-spanning regions
  • Receptor coupled to G protein, and G protein
    stimulates effector (enzyme that promotes
    formation of intracellular second messenger)

25
Guanosine triphosphate
26
  • The G-protein system cycles between on off.
  • When a G-protein-linked receptor is activated by
    binding with an extracellular signal molecule,
    the receptor binds to an inactive G protein in
    membrane.
  • This leads the G protein to substitute GTP for
    GDP.
  • The G protein then binds with another membrane
    protein, often
  • an enzyme,
  • altering its
  • activity and
  • leading to a cellular response.

27
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28
  • The whole system can be shut down quickly when
    the extracellular signal molecule is no longer
    present.

29
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30
G-Protein receptors
  • Diseases such as diabetes and certain forms of
    pituitary cancer, among many others, are thought
    to have some root in the malfunction of G
    proteins
  • G-protein receptor systems are extremely
    widespread and diverse in their functions.
  • They play an important role during embryonic
    development and sensory systems.

31
Ion-Channel receptors
  • Ligand binding changes confirmation of the
    receptor so that specific ions can flow through
    it
  • Ion movement alters the electric potential across
    the plasma membrane
  • found in high numbers on neuron plasma membranes
  • ligand-gated channels for sodium and potassium
  • Also found on the plasma membrane of muscle cells
  • binding of acetylcholine results in ion movement
    and eventual
  • contraction of muscle

32
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33
  • 3.Ligand-gated ion channels protein pores that
    open or close in response to a chemical signal.
  • This allows or blocks ion flow, such as Na or
    Ca2.
  • Binding by a ligand to the extracellular side
    changes the proteins shape and opens the
    channel.
  • Ion flow changes the concentration inside the
    cell.
  • When the ligand dissociates, the channel closes.

34
  • Ligand-gated ion channels are very important in
    the nervous system.
  • EX binding of a neurotransmitter to a neuron,
    allowing the inward flow of Na2 that leads to
    the depolarization of the neuron and the
    propagation of a nervous impulse to adjacent
    cells.

                                    
This colored scanning electron micrograph shows
the synapses, or connections, between two nerve
fibers (in purple) and a nerve cell (yellow). The
picture is magnified 10,000 times.
35
Way Cool Alert!
  • Normally, the enzyme acetylcholinesterase
    converts acetylcholine into the inactive
    metabolites choline and acetate.
  • The devastating effects of nerve agents (Sarin
    gas for example) are due to their inhibition of
    this enzyme, resulting in continuous stimulation
    of the muscles, glands and central nervous system.
  • Botulinus toxin is produced by the anerobic
    bacillus Clostridium botulinum, which may be
    found in improperly canned food, and is one of
    the most potent toxins known.
  • This toxin (the agent responsible for botulism)
    blocks the release of vesicles. This, of course,
    leads to muscle paralysis and, if the diaphragm
    becomes affected, can be fatal.

36
Tyrosine Kinase-linked receptors
Activation Aggregation then phosphorylation
  • Ligand binding results in the formation of a
    receptor dimer (2 receptors)
  • The dimer then activates a class of protein
    called tyrosine
  • kinases
  • This activation results in the phosphorylation
    of downstream
  • targets by these tyrosine kinases (stick
    phosphate groups onto
  • tyrosines within the target protein)

Protein Kinases general name for an enzyme that
transfers PO4- groups from ATP to a protein
37
An individual tyrosine-kinase receptor consists
of several parts
  • an extracellular signal-binding sites
  • a single alpha helix spanning the membrane, and
  • an intracellular tail with several tyrosines.

38
  • When ligands bind to two receptor polypeptides,
    the polypeptides aggregate, forming a dimer.
  • This activates the tyrosine-kinase section of
    both.
  • These add phosphates to the tyrosine tails of the
    other polypeptide.

39
  • The fully-activated receptor proteins activate a
    variety of specific relay proteins that bind to
    specific phosphorylated tyrosine molecules.
  • One tyrosine-kinase receptor dimer may activate
    ten or more different intracellular proteins
    simultaneously.
  • These activated relay proteins trigger many
    different transduction pathways and responses.

40
  • The tyrosine-kinase receptor system is especially
    effective when the cell needs to regulate and
    coordinate a variety of activities and trigger
    several signal pathways at once.
  • Extracellular growth factors often bind to
    tyrosine-kinase receptors.

41
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42
Insulin (Click)
  • Polypeptide hormone that regulates carbohydrate
    metabolism.

43
  • PROJECTS END HERE

44
Step 1 Continued Communication through
diffusion- intracellular receptors
  • Some signal receptors are dissolved in the
    cytosol or nucleus of target cells.
  • The ligands pass through the plasma membrane.
  • Include the hydrophobic steroid and thyroid
    hormones of animals.

45
Action of Lipid-Soluble Hormones (steps)
  1. Hormone diffuses through phospholipid bilayer
    into cell
  2. (Step 1) Binds to an intercellular receptor
    turning on/off specific genes
  3. (Step 3-response) Causing the synthesis of new
    proteins
  4. New proteins alters cells activity

46
  • Testosterone, like other hormones, travels
    through the blood and enters cells throughout the
    body.
  • In the cytosol, they bind and activate receptor
    proteins.
  • These activated proteins enter the nucleus and
    turn on genes that control male sex
    characteristics.

EXAMPLE
47
  • These activated proteins act as transcription
    factors.
  • Transcription factors control which genes are
    turned on which genes get transcribed into
    messenger RNA (mRNA).
  • The mRNA molecules leave the nucleus and carry
    information that directs the synthesis
    (translation) of specific proteins at the
    ribosome.

48
Part 3 Signal-Transduction Pathways
Step 2. In transduction, binding leads to a
change in the receptor that triggers a series of
changes along a signal-transduction pathway.
  • Pathways relay signals from receptors to cellular
    responses-Usually a multi-step pathway
  • Amplification (small number of signal molecules
    can produce a large cellular response)
  • Protein phosphorylation, a common mode of
    regulation in cells, is a major mechanism of
    signal transduction
  • Certain small molecules and ions are key
    components of signaling pathways (second
    messengers-cAMP, Ca2,IP3, GMP )

Insulin Signaling Clip
49
Mr. Anderson Signal Transduction Pathways
50
Pathways relay signals from receptors to cellular
responses
  • Pathway acts like falling dominoes.
  • The signal-activated receptor activates another
    protein, which activates another and so on, until
    the protein that produces the final cellular
    response is activated.
  • The original signal molecule is not passed along
    the pathway and may not even enter the cell.
  • Its information is passed on.

51
Protein phosphorylation is a major mechanism of
signal transduction
phosphorylation
  • The phosphorylation of proteins by a specific
    enzyme (a protein kinase) is a widespread
    cellular mechanism for regulating protein
    activity.
  • Most protein kinases act on other substrate
    proteins, unlike the tyrosine kinases that act on
    themselves.

dephosphorylation
Phosphorylation is the addition of a phosphate
(PO4) group to a protein or a small molecule
Many enzymes and receptors are switched "on" or
"off" by phosphorylation and dephosphorylation.
52
  • Phosphorylation of a protein typically converts
    it from an inactive form to an active form.
  • The reverse (inactivation) is possible too for
    some proteins.
  • A single cell may have hundreds of different
    protein kinases, each specific for a different
    substrate protein.
  • Fully 1 of our genes may code for protein
    kinases.
  • Abnormal activity of protein kinases can cause
    abnormal cell growth and contribute to the
    development of cancer.

ON
protein kinase
phosphorylation
dephosphorylation
protein phosphatases
OFF
53
  • Turning off a signal-transduction pathway -
    protein phosphatases.
  • Rapidly remove phosphate groups from proteins.
  • When an extracellular signal molecule is absent,
    active phosphatase molecules predominate, and the
    signaling pathway and cellular response are shut
    down.

ON
phosphorylation
dephosphorylation
OFF
54
Certain signal molecules and ions are key
components of signaling pathways (second
messengers- cAMP, Ca2,IP3, GMP )
  • Many signaling pathways involve small,
    nonprotein, water-soluble molecules or ions,
    called second messengers.
  • These molecules rapidly diffuse throughout the
    cell.
  • Two of the most important 2nd messangers are cAMP
    and Ca2.
  • Inositol triphosphate (IP3) GMP are others

55
  • Binding by epinephrine leads to increases in
    the concentration of cyclic AMP or cAMP.

cAMP
  • This occurs because the receptor activates
    adenylyl cyclase, which converts ATP to cAMP.
  • cAMP is short-lived as phosphodiesterase converts
    it to AMP.

McGraw-Hill Animation
Epinephrine stimulates glycogen breakdown
56
cAMP -Main purpose activation of protein
kinases. -also used to regulate the passage of
Ca2 through ion channels.
cAMP
cAMP is synthesised from ATP by adenylyl cyclase.
Adenylate cyclase is located at the cell
membranes. It is activated by the hormones
glucagon (does the opposite of insulin) and
adrenaline and by G protein
cAMP controls many biological processes,
including glycogen decomposition into glucose
(glycogenolysis), and lipolysis
57
In response to a signal, a cell may regulate
activities in the cytoplasm or transcription in
the nucleus
  • Ultimately, a signal-transduction pathway leads
    to the regulation of one or more cellular
    activities.
  • This may be a change in an ion channel or a
    change in cell metabolism.
  • EX, epinephrine activates enzymes that catalyze
    the breakdown of glycogen.

Secondary messengers clip- click
58
  • Hormones and other signals can trigger the
    formation of cAMP.
  • Binding by the signal to a receptor activates a G
    protein that activates adenylyl cyclase in the
    plasma membrane.

cAMP
  • The cAMP from the adenylyl cyclase diffuses
    through the cell and activates a
    serine/threonine kinase, called protein kinase A
    which phosphorylates other proteins.

59
  • Certain microbes cause disease by disrupting the
    G-protein signaling pathways.
  • The cholera bacterium, colonizes the small
    intestine and produces a toxin that modifies a G
    protein that regulates salt and water secretion.
  • The modified G protein is stuck in its active
    form, continuously stimulating productions of
    cAMP.
  • This causes the intestinal cells to secrete large
    amounts of water and salts into the intestines,
    leading to profuse diarrhea and death if
    untreated.

cAMP
The toxin acts as an enzyme that changes the G
protein so that it can no longer switch itself
off
60
  • Many signal molecules in animals induce responses
    in their target cells via signal-transduction
    pathways that increase the cytosolic
    concentration of Ca2.
  • In animal cells, increases in Ca2 may cause
    contraction of muscle cells, secretion of some
    substances, and cell division.
  • In plant cells, increases in Ca2 trigger
    responses for coping with environmental stress,
    including drought.
  • Cells use Ca2 as a second messenger in both
    G-protein pathways and tyrosine-kinase pathways.

Ca2.
Ca2
61
Other secondary messengers inositol triphosphate
(IP3)- -stimulates the release of calcium ions
from the smooth endoplasmic reticulum
62
SkipShould be in student projects
63
CELL COMMUNICATION
Part 4 Cellular Responses (step3) to Signals
  • In response to a signal, a cell may regulate
    activities in the cytoplasm or transcription in
    the nucleus
  • Elaborate pathways amplify and specify the cells
    response to signals

64
  • The stimulation of glycogen breakdown by
    epinephrine
  • involves a G-protein-
  • linked receptor,
  • a G Protein adenylyl cyclase and cAMP, and
    several protein kinases before glycogen
    phosphorylase is activated.

Notice The cascading and amplifying effect
65
  • Other signaling pathways do not regulate the
    activity of enzymes but the synthesis of enzymes
    or other proteins.
  • Activated receptors may act as transcription
    factors that turn specific genes on or off in the
    nucleus.

66
Pathways amplify and specify the cells response
to signals
  • Pathways with multiple steps have 2 benefits.
  • They amplify the response to a signal.
  • At each catalytic step in a cascade, the number
    of activated products is much greater than in the
    preceding step.
  • In the epinephrine-triggered pathway, binding by
    a small number of epinephrine molecules can lead
    to the release of hundreds of millions of glucose
    molecules.
  • They contribute to the specificity of the
    response.

67
  • Various types of cells may receive the same
    signal but produce very different responses.
  • For example, epinephrine triggers liver or
    striated muscle cells to break down glycogen, but
    cardiac muscle cells are stimulated to contract,
    leading to a rapid heartbeat.
  • These differences result from a basic
    observation
  • Different kinds of cells have different
    collections of proteins.
  • STRUCTURE AND FUNCTION.

click
68
  • The response of a particular cell to a signal
    depends on its particular collection of receptor
    proteins, relay proteins, and proteins needed to
    carry out the response.

69
  • As important as activating mechanisms are, we
    must say something about inactivating mechanisms.
  • For a cell to remain alert and capable of
    responding to incoming signals, each molecular
    change in its signaling pathways must last only a
    short time.
  • If signaling pathway components become locked
    into one state, the proper function of the cell
    can be disrupted.
  • Binding of signal molecules to receptors must be
    reversible, allowing the receptors to return to
    their inactive state when the signal is released.
  • Similarly, activated signals (cAMP and
    phosphorylated proteins) must be inactivated by
    appropriate enzymes to prepare the cell for a
    fresh signal.

70
Extracellular Signaling Review
  • Signaling molecules are released by signaling
    cells
  • The signal is called the ligand
  • The ligand binds to its specific receptor on a
    target cell
  • This ligand-receptor interaction induces a
    conformational or shape-change in the receptor
  • Produces a specific response - called the
    cellular response
  • Can include a vast array of compounds
  • e.g. small amino acid derivatives, small
    peptides, proteins

71
Cell-to-cell communication by extracellular
signaling usually involves six steps
  • (1) synthesis of the signaling molecule by the
    signaling cell
  • (2) release of the signaling molecule by the
    signaling cell
  • (3) transport of the signal to the target cell
  • (4) detection of the signal by a specific
    receptor protein receptor-ligand specificity
  • (5) a change in cellular metabolism, function, or
    development cellular response
  • triggered by the receptor-ligand complex
    specific to the ligand-receptor complex
  • (6) removal of the signal, which usually
    terminates the cellular response degredation of
    ligand
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