6 Cell Communication

Lecture Outline

I. Cell Signaling: An Overview

A. Examples of cell signaling

1. Protists: slime molds secrete cAMP to form a slug-shaped colony

2. Plants: diseased maples send signals to uninfected trees nearby

B. Definition of cell signaling

1. Mechanisms by which cells communicate

2. Two mechanisms

a) Physically close cells actually combine

b) Chemical signals are the most common mechanism

C. Develop and functioning require communication between cells

1. Plants and animals utilize hormones for internal communication

a) Hormones affect target cells

b) Target cells receiving information is called reception

c) Transduction is the conversion of an extracellular signal to an intracellular signal

2. Most animals have neurons that transmit electrical and chemical signals, and also communicate internally and detect the external environment

II. Sending Signals: Types of signals

A. Cell to cell (direct contact)

1. Growth factors stimulate cell division and normal development

2. Histamines cause allergic reactions by dilating blood vessels and making capillaries more permeable

B. Local Regulators

1. Prostaglandins have diverse affects in local tissues, such has smooth muscle relaxation

2. Nitric oxide is a gas that causes effects in plants and animals

C. Neurons

1. Signal each other with neurotransmitters such as acetylcholine

D. Distant cells

1. Endocrine glands secrete hormones that travel through the circulatory system of animals.

III. Reception

A. Cells are genetically programmed to receive and send signals

B. Receptors

1. Large proteins or glycoproteins that bind with signaling molecules

2. A ligand is a signaling molecule that binds to a specific receptor

a) Most ligands are hydrophilic and bind to protein receptors on the surface of target cells

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b) Hydrophobic molecules move through the plasma membrane and bind to intracellular receptors

3. Three domains of a receptor

a) First domain: External-docking site for a signaling molecule

b) Second domain: Extends through the plasma membrane

c) Third domain: A A tail@ that extends into the cytoplasm and transmits the signal inside the cell

4. Reception is highly selective

5. Different types of cells can have different types of receptors

6. Some receptors are designed to respond to signals other than chemical ones

a) Vision in animals involves the receptor rhodopsin and is activated by light

b) Plants and animals have phytochromes that are activated by light

C. Cells regulate reception by increasing or decreasing the number of receptors

D. Three types of cell surface receptors

1. Ion channel-linked receptors (ligand-gated channels)

a) Ion channel opens or closes in response to ligand (signaling molecule)

b) Neurotransmitters can be the signaling molecules, initiating a nerve impulse by depolarizing the next neuron= s membrane

2. G protein-linked receptors

a) Transmembrane proteins

b) Couple specific signaling molecules to signal transduction pathways inside the cell.

c) About 60% of prescribed medications act on these receptors

3. Enzyme-linked receptors

a) Function as enzymes or act on enzymes

b) Examples

(1) Tyrosine kinase receptors bind to growth factor signal molecules.

(2) In plants, protein kinase receptors in the plasma membrane bind with brassinolide, a steroid hormone that regulates cell division, cell elongation, and flower development

E. Intracellular receptors (transcription factors)

1. Proteins that regulate the expression of specific genes

IV. Signal Transduction

A. Ion channel-linked receptors open or close channels

1. Ion channel gates remain closed until a ligand binds to the receptor

2. GABA is a neurotransmitter that binds to ligand-gated chloride ion channels in neurons

3. GABA causes negative ions to rush in, inhibiting neural signaling

B. G protein-linked receptors initiate signal transduction

1. G protein linked receptors activate G proteins

2. These are found in yeasts, protists, plants, and animals

3. They are involved in plant and animal hormone action

4. Inactive G protein consists of three subunits

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5. When activated, G protein initiates signal transduction by binding with a specific protein in the cell

6. Most often, the G protein relays information from an extracellular first messenger (ligand) to an intracellular second messenger

C. Second messengers are intracellular signaling agents

1. These are ions or small molecules that relay signals inside the cell

2. Second messengers are often produced in large quantities which amplifies the signal.

3. Mechanism of action

a) Regulate enzymes

b) Bind to ion channels, either opening or closing them

c) Start a signaling cascade (chain of molecules relaying a signal)

4. Examples of second messengers

a) Cyclic AMP

b) Phospholipids

c) Calcium ions

D. Enzyme-linked receptors function directly

1. Most enzyme-linked receptors are tyrosine kinases

2. Tyrosine kinase receptors activate several different signal transduction pathways

a) Ras protein pathway

E. Many activated intracellular receptors are transcription factors

1. Transcription factors are hydrophobic signaling molecules that diffuse across target cell membranes

a) The ligand receptor complex binds to a specific region of DNA and activates or represses a gene

b) If gene activation occurs, messenger RNA is produced

2. Nitrous oxide is a gaseous signaling molecule

a) NO binds to guanylyl cyclase

b) Guanylyl cyclase catalyzes cyclic GMP production

c) cGMP causes smooth muscle relaxation

d) Sildenafil (Viagra) inhibits the enzyme that catalyzes cyclic GMP breakdown

(1) Results in cGMP concentration increasing in the erectile tissue of the penis

(2) Leads to a sustained erection

F. Scaffolding proteins increase efficiency by keeping the molecules involved in relaying a signal close together

1. Prevents enzymes involved from being used by other pathways

G. Signals can be transmitted in more than one direction

V. Responses to Signals

A. Three categories of responses

1. Ion channels open or close

a) Neurotransmitters in animals

(1) Acetylcholine binds with a neuron or muscle receptor and opens an ion channel, allowing the passage of sodium and potassium


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(2) Serotonin works indirectly through G proteins and cAMP to close potassium ion channels, leading to neural impulse transmission

2. Enzyme activity is altered

a) Directly

b) Altered by components of the pathway

3. Specific gene activity is turned on or off

a) Ras pathway

B. The response to a signal is amplified

1. Example: epinephrine

a) Epinephrine is released by the adrenal glands in response to stress

(1) Increases heart rate, blood flow to skeletal muscle, and blood glucose concentration

b) Epinephrine binds to a G protein-linked receptor

c) Receptor changes shape and activates a G protein

d) Many G proteins can be activated ((signal amplification)

e) The G proteins can be reactivated after doing there job

f) Each G protein activates adenylyl cycles molecule

g) Each adenylyl cyclase produces many cAMP molecules (signal amplification)

h) Each cAMP produces a protein kinase that phosphorylates many protein molecules (signal amplification)

C. Signals must be terminated

1. Returns signal transduction components to their inactive states

2. Cyclic AMP example

3. Cholera is a disease of unterminated cell signals

a) Cholera is caused by a bacterium in infected human feces and is spread in feces contaminated water

b) Cholera bacterium releases a toxin that activates G protein in the lining of the intestines, preventing G protein from switching off

c) G protein continues to stimulate adenylyl cyclase to make cAMP

d) This allows a large flow of chloride ions into the intestine

e) Treatment is replacing lost fluid

f) If untreated, it can cause death

VI. Evolution of Cell Communication

A. Similarities in cell signaling among organisms suggest that the molecules and mechanisms must be very old

B. Since these pathways have survived, they have positively affected organism natural selection