Second messenger system

Cells use tiny signals to talk.

A signal hits the outside of a cell. It cannot get inside alone. So, it makes a second signal. This new signal moves inside the cell. It tells the cell what to do.

These new signals are very important. They can help a cell grow. They can even help a cell move.

Some signals are like tiny bits of gas. Other signals can be found in water. Some signals stay near the cell edge.

One signal is called calcium. It helps your muscles move. It also helps cells join together.

Scientists found these tiny signals. They won prizes for their work. Now we know how cells talk!

Cells need to talk to each other. They use tiny signals to send messages.

Sometimes, a signal stays outside the cell. We call these first messengers. These are often hormones or chemicals. Many first messengers cannot enter the cell. They cannot pass through the cell's outer layer. This is why the cell needs a second way to hear the message.

When a first messenger hits a cell, it triggers a second signal. We call these second messengers. They carry the message deep inside the cell. This set of steps is called signal transduction.

Second messengers can be different types. Some can dissolve in water. These are called hydrophilic molecules. Others do not dissolve in water. We call these hydrophobic molecules. Some messengers are even gases.

One important messenger is calcium. Calcium ions can help muscles move. They also help cells join together. These signals can make a small message much stronger. This helps the cell respond quickly.

Scientists Earl Wilbur Sutherland Jr., Martin Rodbell, and Alfred G. Gilman studied these systems. They won Nobel Prizes for their work. They found how signals make liver cells turn sugar into energy.

Cells must receive messages to know how to work. Sometimes, a signal stays outside the cell. We call these first messengers. These are often hormones or neurotransmitters. Some signals, like steroid hormones, can enter a cell easily. However, many first messengers are hydrophilic. This means they do not cross the cell's outer layer. Because they cannot enter directly, the cell needs a special way to hear them.

To fix this, cells use a second messenger system. This is a way to turn an outside signal into an inside signal. This process is called signal transduction. First, a messenger binds to a receptor on the cell surface. This binding causes the receptor to change its shape. This change can activate a G-protein. A G-protein acts like a transducer to pass the message along. It swaps a molecule called GDP for one called GTP. This causes a part of the protein to break free and move. This moving part then creates the second messenger inside the cell.

Scientists spent many years studying these tiny signals. Earl Wilbur Sutherland Jr. discovered second messengers. He saw that epinephrine could make liver cells turn glycogen into glucose. He found that epinephrine alone could not do this task. It needed a second messenger called cyclic AMP to work. For this great work, he won the Nobel Prize in 1971. Later, Martin Rodbell and Alfred G. Gilman worked out the details. They won the Nobel Prize in 1994 for their discoveries.

Second messengers come in three main types. Some are hydrophobic, which means they do not dissolve in water. These stay near the cell membrane. Others are hydrophilic, meaning they dissolve in water. These move through the cytosol, or the fluid inside the cell. Some second messengers are even gases, like nitric oxide. One very important messenger is calcium. Calcium ions are often stored in parts of the cell called organelles. When a signal arrives, these ions are released quickly.

These systems help the cell react to its world. Second messengers can make a small signal much stronger. This is called amplification. For example, a signal can trigger a large cascade of proteins. This helps the cell grow, move, or even survive. Calcium is a great example of this work. It helps with muscle contraction and the release of neurotransmitters. By using these messengers, a single tiny signal can change how a whole cell behaves.

Cells must constantly respond to their environment to survive and function. They do this through intercellular signaling, which involves sending messages between cells. These signals are categorized as autocrine, juxtacrine, paracrine, or endocrine depending on how far they travel. A signal begins with a first messenger, which is an extracellular molecule like a hormone or a neurotransmitter. Many of these first messengers, such as epinephrine or serotonin, are hydrophilic. This means they are water-soluble and cannot cross the cell's fatty phospholipid bilayer. Because they cannot enter the cell directly, they require a second messenger system to pass the message inside.

This process of converting an external signal into an internal one is called signal transduction. It often begins when a ligand, or signaling molecule, binds to a cell surface receptor. This binding causes a conformational change, which is a change in the shape of the receptor. In many systems, this change activates a G-protein, which acts as a transducer. A G-protein consists of three subunits: alpha, beta, and gamma. When the receptor changes shape, it allows the G-protein to exchange a GDP molecule for a GTP molecule on its alpha subunit. This exchange causes the alpha subunit to break away from the other subunits to move along the membrane.

Once the alpha subunit is free, it contacts a primary effector. This effector then triggers the production or release of second messengers. These small molecules, such as cyclic AMP or calcium, can then activate various proteins like protein kinases or ion channels. This creates a signaling cascade that continues the message throughout the cell. A major advantage of this system is amplification. A single first messenger can trigger a cascade that greatly increases the strength of the original signal. For example, RasGTP signals can link to the mitogen-activated protein kinase (MAPK) cascade. This amplifies the activation of transcription factors like Myc and CREB, which help the cell grow.

Second messenger molecules are classified into three distinct types based on their chemical properties. The first type is hydrophobic molecules, such as diacylglycerol and phosphatidylinositols. These are water-insoluble and remain associated with the cell membrane. They diffuse into the intermembrane space to regulate membrane-associated proteins. The second type is hydrophilic molecules, which are water-soluble. Examples include cyclic AMP (cAMP), cyclic GMP (cGMP), inositol triphosphate (IP3), and calcium (Ca2+). These molecules move through the cytosol, the fluid inside the cell. The third type includes gases like nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S). These gases are unique because they can diffuse through both the cytosol and cellular membranes.

One specific and vital pathway is the phosphoinositol signaling pathway. This pathway often begins when primary messengers like acetylcholine or oxytocin bind to G-protein-coupled receptors (GPCRs). This binding triggers the alpha subunit to activate an enzyme called phospholipase C. This enzyme then hydrolyzes a membrane molecule known as PIP2. This chemical reaction produces two important second messengers: diacylglycerol (DAG) and IP3. While DAG stays in the membrane, IP3 travels to the endoplasmic reticulum (ER). IP3 binds to calcium pumps on the ER, causing the release of Ca2+ into the cytoplasm.

Calcium ions serve as a critical example of a second messenger. They are often stored in organelles like the endoplasmic reticulum until they are needed. When an active G-protein opens calcium channels, these ions flood into the cell. Calcium is responsible for many essential physiological functions, such as muscle contraction, fertilization, and the release of neurotransmitters. The production and destruction of second messengers can be highly localized. This allows the cell to control exactly where and when a signal is active. Some messengers are synthesized by enzymes like cyclases, while others are released through ion channels.

Our understanding of these systems comes from decades of dedicated research. Earl Wilbur Sutherland Jr. discovered second messengers and won the 1971 Nobel Prize in Physiology or Medicine. He observed that epinephrine stimulates liver cells to convert glycogen into glucose. He realized that epinephrine could not do this alone; it required the second messenger cyclic AMP. Later, Martin Rodbell and Alfred G. Gilman won the 1994 Nobel Prize for working out the detailed mechanisms of these systems. Their work helped explain how cells turn tiny external signals into massive internal changes.