Cell Signaling and Communication – Part 2

The process that converts a signal on a cell’s surface into a cellular response involved a series of steps that are collectively called signal transduction pathway. Interestingly, the details of signal transduction in yeast and mammals are quite similar, showing that these processes have been highly conserved through the process of evolution. As well, similarities between signal transduction in plants and bacteria show that this is an ancient process that has evolved along with live on Earth.

Signal Transduction Pathway – 3 Steps

1. Reception – the signal molecule attaches to a receptor protein or to a sugar associated with a receptor glycoprotein. In doing so, the signal molecule is acting as a ligand, a molecule that attaches to another molecule due to complimentary shape and chemical nature. When the signal molecule attaches, the receptor undergoes a conformational change (allosteric change) and its shape changes.

Note: Some receptors may be activated by environmental factors such as light, pH, or temperature.

2. Transduction – relay molecules called second messengers become activated and start a series of reactions in the cytoplasm that amplify. These reactions get more intense at each step and are called a cascade.

3. Cellular Response – the final molecule in the signal transduction pathway will lead to a nuclear response and/or a cytoplasmic response within the cell.

Regulate gene expression in the nucleus by turning a gene on or off. Activate an enzyme to catalyze a specific chemical reaction. Open or close a gated channel protein within the cell membrane.

Campbell (8th ed.) p209-225

Cascade of Events – each successive step in the signal transduction pathway gets stronger.

Three Types of Membrane Protein Reception

1. G-Coupled Reception

Approximately 60% of all drugs currently on the market interact with this type of receptor protein. As a result, it is important to understand the different ways in which it can function in order to predict the affect of a drug.

2. Tyrosine-Kinase Reception

Abnormalities in the functioning of this receptor have been linked to uncontrolled cell division associated with cancer. Understanding the functioning of this receptor could lead to drugs that inhibit tumor growth.

3. Gated Channel Reception

Muscle and nerve cells as well as other cells in animals use these types of channels to regulate their functioning. Understanding the nervous system and muscular system involves understanding the action of these receptors.

G-Coupled Reception

1. No Signal Molecules Present

A signal molecule (ligand) is not bound to the receptor protein. As well, the G-protein is in its inactive form (bound to GDP) and the enzyme is also inactive.

2. Attachment of a Messenger Molecule and G-Protein Activation

The signal molecule (ligand) attaches to the receptor protein in the membrane and activates it. The activated receptor causes GTP to bind to the G-protein and for GDP to be released from the G-protein. These two changes activate the G-protein.

3. Activation of the Enzyme that Produces cAMP

The signal molecule (ligand) is released from the receptor and the receptor is left in its inactive form. This occurs after the activated G-protein starts moving along the surface of the membrane to the enzyme (adenylyl cyclase). When the G-protein binds to the enzyme, it activates it by changing its conformation (shape). The activated enzyme then causes ATP to be converted into cAMP.

The cAMP produced is the second messenger molecule that is used to start the cellular response (transduction).

Note: If the enzyme activated is guanylyl cyclase, cGMP is the second messenger molecule that is formed.

Adenylyl Cyclase

cAMP starts signal transduction by activating kinase A

4. Return of G-protein to Resting Position

The G-protein acts as a GTPase enzyme and eventually converts the GTP it is carrying back to GDP and a free phosphate molecule (Pi). When this occurs, the G-protien reverts back to its inactive state and moves back to the receptor protein so that it is ready for future responses to signal molecules.

Note: The sequence of diagrams shown provides an account of the G-Receptor system first discovered by Earl Sutherland in regard to his work on how epinephrine (adrenaline) leads to glycogen breakdown in cells. However, there are variations in the molecules involved and modes by which they function. Two examples are bulleted below that help show that not all G-receptor systems are the same and that decoding the functioning of these systems is difficult.

The enzyme that is activated by the G-protein may differ. If, guanylyl cyclase is activated cGTP will be produced rather than cAMP.

Some G-proteins may inhibit rather than activate certain enzymes. In such cases attachment of a signal molecule to a receptor and subsequent activation of a G-protein may cause an enzyme to be turned off and for signal transduction to cease functioning rather than be turned on.

Write the function for each of the molecules involved in the G-Protein-Linked Receptor system using your textbook as a reference source.

Signal Molecule –

Receptor Protein –

GTP –

G-Protein –

Adenylyl Cyclase –

ATP –

cAMP –

Protein Kinase -

Diagrammatic Modeling

Vibrio cholerae is the bacterium that causes the disease cholera in humans. Use the textbook to obtain information on how the toxins produced by the bacterium affect the functioning of G-Reception in human cells and make a series of annotated diagrams that shows the toxin’s action.

Textbook – Campbell (8th ed.) – p 217

Tyrosine-Kinase Reception

1. Signal Molecule Not Attached

The tyrosine kinase receptor exists as two separate proteins in the cell membrane before signal molecules (ligands) attach to each of the proteins. The tyrosine amino acids that extend from the cytoplasmic regions of the proteins are not phosphorylated at this point.

2. Signal Molecules Attach to Receptors

Each of the proteins must bind to a signal molecule (ligand) in order for them to join together (dimerization) and become activated.

3. Phosphorylation of Tyrosine Regions

The tyrosine amino acids that are in the cytoplasmic region of the receptor protein are phosphorylated, and the receptor protein becomes fully activated.

4. Relay Proteins are Activated and Start Signal Transduction

Inactive relay proteins in the cytoplasm attach to the phosphorylated tyrosine regions of the receptor and become activated. Each of the relay proteins is then capable of starting its own cellular response (signal transduction).

Ligand Gated Channel Reception

1. Signal Molecule Not Attached

When the signal molecule (ligand) is not attached to the ligand-gated channel protein, the channel for ion movement is closed.

2. Signal Molecule Attaches and Channel Opens

When the signal molecule (ligand) attaches to the channel receptor protein, it undergoes a conformational change (shape change) and ions can move through it. The ions act as second messenger molecules within the cell and trigger a cellular response.

Note: Calcium ions (Ca2+) are a very important second messenger molecule in many cells and are typically found in low concentration in the cellular cytoplasm when a cell has not been triggered by a signal molecule.

Compare and Contrast

How is the Tyrosine-Kinase Receptor system different from the G-Linked-Protein Receptor system?

How do Ion-Gated Channels differ from both G-Linked-Protein Receptor systems and Tyrosine-Kinase Receptor systems?

Cytoplasmic Protein Reception

In some cases, signal molecules are small and/or lipid soluble. As a result, they can pass through the cell membrane and enter the cytoplasm. This is true for steroid hormones, which include testosterone, estrogen, and cortisone. Once inside the cell, these signal molecules bind to cytoplasmic receptor proteins to from a hormone-receptor complex. This complex can then enter into the nucleus and influence gene expression.

As well, many receptors that are stimulated by environmental factors are located in the cytoplasm. For example, the phytochrome protein found in photosynthetic plant cells is found in the cytoplasm and is activated by light. It then is responsible for a series of signal transduction responses.

Signal Transduction

Technically, signal transduction starts when a second messenger molecule such as cAMP, cGMP, IP3, DAG, or Ca2+ acts as an activated relay molecule to activate the first kinase protein (kinase A) in a series of reactions.

Once the first kinase protein has been activated, it facilitates the phosphorylation of numerous second kinase proteins so that they become activated. The second kinase proteins facilitate the phosphorylation and activation of numerous third kinase molecules and so on. This series of phosphorylation reactions increases in strength and is called the phosphorylation cascade.

At the end of the cascade, a protein that can alter cellular functioning is activated and this causes a cellular response to occur.

Cyclic AMP (cAMP) is a second Messenger molecule that functions as an activated

relay molecule.

kinases activate

protein phosphatases

(PP) deactivate

Types of Transduction Responses

1. Transduction may lead to a single response or a branched response that leads to activation of two or more pathways.

2. Different transduction pathways that are operating at the same time may interact with each other. As well, second messenger molecules can stimulate or inhibit receptors that are stimulated by a different signal molecule, especially gated ion channels.

3. When a receptor is exposed to its own signal molecule for an extended period of time, it may become desensitized by one of the second messenger molecules activated during transduction. Desensitization means that signal molecules will have less effect on the receptor, and is a type of negative feedback that occurs to slow the response.

4. Many signal transduction pathways operate at the same time within cells and have complex interactions with each other.

For example, the activated cytoplasmic receptor (phytochrome) in this plant cell is stimulating another receptor (calcium leak channel protein) as well as starting its own signal transduction pathway via kinase activation by cGMP.

Desensitizationof Receptor

Model of what a Signal Transduction Pathways Actually Looks Like

Molecules of different transduction responses may interact with each other leading to complex interactions with complex regulatory mechanisms. Mapping and understanding signal transduction pathways is a current focus of cell biology research.

Cellular Responses

At the end of the cascade of phosphorylation reactions involved in transduction, a protein(s) is activated that causes a cellular response. Nuclear responses involve the alteration of gene expression, while cytoplasmic responses involve the activation of an enzyme or the opening of an ion channel.

Example 1 – Yeast Cells (Cytoplasmic Response - Enzyme Activation)

The attachment of a mating factor protein to a receptor protein in the yeast cell membrane triggers a signal transduction pathway that ultimately leads to the activation an enzyme that forms microfilaments. The microfilaments then cause membrane to change shape and protrude so that two adjacent cells of opposite mating type may fuse together.

Example 2 – Mammalian Cells (Cytoplasmic Response –Enzyme Activation)

The hormone epinephrine (adrenaline) can bind to membrane receptors and start signal transduction. The ultimate cellular response that results is the activation of glycogen phosphorylase, the enzyme which functions to break down stored glycogen into glucose for metabolic use.

Example 3 – Human Cells (Nuclear Response - Gene Expression)

Growth factors bind to receptors in the cell membrane and triggers signal transduction reactions. The final kinase protein that is activated enters the nucleus and activates transcription factor proteins. This leads to the expression of a gene that produces a protein involved in cell growth and division.

Example 4 - Plant Leaves (Nuclear Response - Gene Expression)

In this case, sunlight is the environmental stimulus that activates phytochrome proteins to set the signal transduction in motion. Ultimately, transcription factor proteins that are responsible for turning on the expression specific genes within the DNA are activated. The proteins produced by these activated genes are the enzymes responsible for producing molecules involved in the de-etiolation (greening) process.

Campbell (8th ed.) - p822-823

Example 5 – Animal Embryonic Development (Nuclear Response - Gene Expression)

Signal molecules are released by cells within the embryo which allow for communication and coordination of development. Signal molecules bind to receptors of neighboring cells and signal transduction occurs to result in specific gene expression. This process in which a cell releases signal molecules to influence a neighboring cell’s specialization is called induction.

In the diagram below, an embryonic precursor cell (unspecialized embryo cell) was triggered by signals from neighboring cells to develop into a myoblast (muscle cell). This involved signal reception, transduction, and a cellular response. The final molecule activated by transduction was the transcription factor called MyoD, which then stared a nuclear response that involves expression of specific genes and production of proteins necessary for differentiation (specialization) into a muscle cell.

Example 6 – Apoptosis in the C. elegans Worm (Nuclear Response – Gene Expression)

When a cell is functioning normally, the Ced-9 protein inhibits activity of proteins involved in a signal transduction pathway that leads to the expression of proteins that cause blebbing and apoptosis (cell death). However, when a membrane receptor comes in

contact with a death signaling molecule, the Ced-9 protein is inhibited and it no longer functions to inhibit the signal transduction pathway. With the signal transduction reactions turned on, proteins (proteases and nucleases) that are needed for blebbing and apoptosis are produced.

Final Protein Activated by Transduction

Nuclear response involves changes in gene expression.

Signals from neighboring cells trigger differentiation.

Campbell (8th ed.) – p367-369

Apoptosis in more advanced animals involves several different pathways and is more complex than in C. elegans. Nonetheless, gene expression is altered so that proteins responsible for degrading cellular components are produced.

Human White Blood Cells – Old Macrophage Lysis

Normal Cell Blebbing occurs during Apoptosis Mouse Paw Development – Apoptosis is responsible for the removal of cells that

form webbing between digits.

Note: Apoptosis is a critical mechanism for correct development in almost all animals. Cells are laid down and then removed by apoptosis during the development process.

Neurodegenerative Diseases (Alzheimer’s Disease and Parkinson’s Disease) -Apoptosis is of nerve cells is thought to contribute to loss of functional nerve tissue.

Cancer – Apoptosis does not function in cancer cells which continually divide and are immortal.

Note: Mechanisms for apoptosis have high degree of similarity in all animals. This shows that apoptosis has been conserved in the process of evolution and has been an important aspect of animal development.

Signal Transduction and Disease

Changes that lead to unnecessary activation or suppression of a signal transduction pathway can lead to disruption of homeostasis and disease. To counter these changes, drugs may be used to activate, suppress, or block a signal transduction pathway. As more signal transduction pathways are mapped, the use of drugs engineered to regulate signal transduction pathways will increase.

Problem - Signal transduction pathway is too active or has been turned-on when it should not be turned-on.

Solution – Administer drugs that serve as inhibitors and block either the receptor protein or any of the kinase proteins involved in transduction.

Example - Heart Disease

Problem – When adrenaline is released from the adrenal glands in times of stress, it binds to receptors found mainly in the smooth muscle cells comprising the blood vessels. The binding of adrenaline leads to a response that involves contraction of the smooth muscle cells (vasoconstriction) and increased blood pressure.

Drug Therapy - Alpha-blocker and beta-blocker molecules attach to the same receptors to which adrenaline binds, but do no not lead to transduction and a response. As a result, alpha-blockers and beta-blockers diminish the affect of adrenaline when it is present due to stress and reduce vasoconstriction to keep blood pressure more stable.

Problem – Signal transduction is not functioning or is functioning too slowly.

Solution - Administer drugs that stimulate receptors or amplify action of specific molecules involved in transduction.

Example - Diabetes

Problem – Beta-cells in the Islets of Langerhans glands that lie on top of the pancreas fail to make sufficient insulin. Insulin is a stimulatory molecule for the signal transduction pathway that leads to cellular glucose intake.

Drug Therapy – Administer artificial human insulin (produced by genetic engineering techniques) at specific times of the day to regulate blood sugar levels

Article - Signal Transduction as a Drug-Discovery PlatformNature Biotechnology 18, IT37 - IT39 (2000)

Abstract - Mapping the key signaling molecules in biochemical pathways will be central to future drug discovery efforts.