Openstax Biology Chapter 9

Intercellular Signaling

Communication between two cells

Intracellular Signaling

Communication within a cell

Ligand

A molecule that binds to another molecule; a type of signaling molecule

Paracrine Signaling

A cell targets another nearby cell

These signals move through the extracellular matrix using diffusion.

Cause quick, short responses

In order the signals local, enzymes quickly degrade them, so they cannot travel any distance and they may only impact nearby cells.

An example of this is two nearby nerve cells communicating with one another. One nerve cell releases neurotransmitters (the ligand/signal molecule). These neurotransmitters travel a short distance to the other neuron.

The neurotransmitters that do not get used build up in the synapse (the area in between the two nerve cells) and either get degraded by enzymes or reabsorbed by the cell that released the neurotransmitters (reuptake).

Local concentrations of ligands (neurotransmitters) are considered high in the synapse

Endocrine Signaling

A cell targets a cell over a great distance using the bloodstream

These signals are released by endocrine cells (thyroid gland, hypothalamus, pituitary gland)

They produce a slow response but have a long-lasting effect

The ligands released are called hormones

The ligands (hormones) are present in low concentration because they must travel through the bloodstream so they are spread throughout the body.

Autocrine Signaling

A cell targets itself

The signaling cell that releases the signal/ligand can also bind to the ligand

This type of signaling typically happens in the early stages of organism development to ensure cells develop into the proper type of cell.

Autocrine signaling also regulates pain and inflammation responses.

Autocrine signaling is also involved in programmed cell death (apoptosis) - incase it is infected with a virus which may kill the virus

Signaling Across Gap Junctions

Gap junctions connect the plasma membrane of neighboring cells in plants and animals

The ligand that travels in between gap junctions is called an intracellular mediator

Intracellular mediators include calcium ions that are small enough to move through cells; large things like proteins and DNA are too big to fit through

Internal Receptors

They are also called intracellular or cytoplasmic receptors

They are found in the cytoplasm of a cell and respond to hydrophobic ligands that can travel across the plasma membrane

Many of these bind to proteins that regulate mRNA synthesis (transcription)

Cell-Surface Receptors

They are also known as transmembrane receptors

They are membrane-anchored integral proteins that bind to external ligands; the ligands do not need to enter cell to do their job

They convert extracellular signals into intercellular signals

Since these receptors are important to normal cell functioning, a malfunction in any one of them could have severe consequences - been linked to hypertension, asthma, heart disease and cancer

Has three main components:

-An external ligand-binding domain (extracellular domain)

-A hydophobic membrane-spanning region

-An intercellular domain inside the cell

Ion Channel-Linked Receptor

A type of cell-surface receptor

Binds a ligand and then opens a channel in the membrane that lets specific ions pass through the membrane

G-Protein Linked Receptor

A type of cell-surface receptor

Binds a ligand and activates a G-Protein (membrane protein)

This G-Protein then interacts with either an ion channel or an enzyme in the membrane

All G-protein linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-Protein binding site

Cell signaling using G-Protein linked receptors involves a cyclic series of events

Pathogenic bacteria can release poisons that interupt g-protein linked receptors and lead to illnesses like pertussis, botulism and cholera.

Enzyme-Linked Receptors

A type of cell-surface receptor

They have intracellular domains that are associated with an enzyme, in some cases, the domain itself is an enzyme

When a ligand binds to the extracellular domain, a signal is transferred through the membrane, activating the enzyme

An example is the tyrosine kinase receptor which transfers phosphate groups to tyrosine.

Small Hydrophobic Ligands

Can directly diffuse through the plasma membrane and interact with internal receptors

Most important ligand is the steroids including estradiol (estrogen), testosterone and cholesterol.

Water-Soluble Ligands

Are polar and cannot pass through membrane without assistance

Most bind to extracellular domains of cell-surface receptors

Includes peptides and proteins

Nitric Oxide

A gas that acts as a ligand which can diffuse directly across the plasma membrane

Interacts with receptors in smooth muscle and causes the relaxation of muscle tissues

Nitroglycerin triggers the release of nitric oxide and causes blood vessels to dilate; it has also been used for erectile dysfunction in Viagra

Signal Transduction

When a signal continually gets sent more than just once

Dimer / Dimerization

When a ligand binds to a receptor, it can sometimes cause two receptors to bind to one another (often identical) and create a stable complex (dimer)

This binding happens to bring the intracellular domains close enough to activate themselves

Signal Integration

When signals from two or more different cell-suface receptors merge together and activate the same response in the cell

Phosphorylation

Adding a phosphate group to a molecule

This is done by an enzyme called a kinase

Second Messengers

Small molecules that propagate a signal after it has been initiated

They help spread the signal through the cytoplasm (over distance)

Calcium ions are a common second messenger

-in the pancreas, calcium signaling releases insulin

-in muscle cells, increase in calcium leads to muscle contractions

Another common second messenger is cyclic AMP (cAMP).

It is made by the enzyme adenylyl cyclase and made from ATP

cAMP binds and activates an enzyme called cAMP-dependent kinase (A-kinase), which regulates metabolic pathways

Other second messengers:

Diacylglcyerol (DAG)

Inositol triphosphate (IP3)

Gene Expression

The protein ERK (a MAP kinase), regulates protein translation in the nucleus

Inhibitor

A molecule that binds to a protein and prevents it from functioning or reduces its function

Increase in Cellular Metabolism

The activation of β-adrenergic receptors in muscle cells by adrenaline leads to an increase in cyclic AMP (cAMP) inside the cell.

This starts a chain reaction which leads to enzymes being produced that break down glycogen into glucose so that the muscles have glucose to utilize

Growth Factors

Ligands that promote cell growth

Apoptosis

Programmed cell death

Allow cells to die in a manner that prevents the release of potentially damaging molecules contained within the cells

An example is the webbing formed between the toes and fingers during embryo development. Apoptosis triggers the destruction of these cells as the embryo grows so that an individual does not have webbed feet or hands.

Termination of the Signal Cascade

Methods to stop signals:

Degrade the ligand or remove it so that it can no longer access its receptor

Phosphatases are enzymes that remove the phosphate group attached to proteins by kinases (dephosphorylation)

Phosphodiesterase degrades cyclic AMP (cAMP) into AMP

Quorum Sensing

A method for helping bacterial determine when to signal

Bacteria primarily signal when they sense a larger number of other cells present in the environment

It uses autoinducers as signal molecules.

Autoinducers are signal molecules secreted by bacteria to communicate with the same type of bacteria

Autoinducers only work on other cells and not on the cell that created and released the signal