5.6 Signalling Across the Plasma Membrane

It is vital for individual cells to be able to interact with their environment. This is true for both unicellular organisms and complex multicellular organisms. Thus, although the plasma membrane keeps lots of molecules out of the cell, information must be able to cross this barrier. To properly respond to external information, cells have developed complex mechanisms of cell signalling, which we will explore in this chapter section.

Steps of Cell Signalling

Cell signalling is the process of detecting a change in a cell’s environment, and generating an appropriate response within the cell. That change may be chemical (e.g., the presence of a hormone or nutrients), or more broad (e.g., a change in temperature or osmolarity). For simplicity, this chapter section will focus on how cells interpret and respond to chemical signals. In later chapters, we will see examples of how cells respond to other types of stimuli. Regardless of the type of environmental change, cell signalling usually involves four steps (Figure 5.32): signal reception, signal transduction, cellular response, and termination of the signal cascade. The following subsections will discuss each step in greater detail.

Step 1: Signal Reception

Signal reception is required to detect a change in conditions in or around a cell via a change in activity of a receptor protein. Receptors are usually proteins and may be inside the cell or within the cell membrane (e.g., transmembrane proteins). Because proteins are stabilized by weak interactions, anything that alters these weak interactions can change the protein’s structure and activity. Several environmental factors can alter the weak interactions in receptor proteins such as temperature and pH. The other main factor that can alter receptor protein structure is the binding of a chemical. Chemicals that bind to receptor proteins are called ligands (Figure 5.33). Both receptors and ligands will be discussed in more detail later in this chapter section. For now what you need to know is that when a ligand binds a receptor, this causes a change in the conformation (shape) of the receptor, which then leads to the next step of cell signalling: signal transduction.

Step 2: Signal Transduction

Signal transduction is the process of converting a signal (e.g., ligand) that interacts with a receptor into intracellular messages that cause changes inside the cell. Signal transduction pathways vary substantially (depending on the cell type, receptor, etc.), but there are a few mechanisms that are common to many transduction pathways. For example, many transduction pathways involve the production of second messengers within the cell. We will explore these later in the chapter, but second messengers tend to be small molecules that can be produced in abundance inside the cell, allowing the cell to amplify the original signal (caused by a “first messenger”).

A generalized signal transduction pathway is shown in Figure 5.34. In this example, ligand binding to the receptor activates an enzyme that catalyzes the formation of many copies of the second messenger. The second messenger then activates additional steps in the signalling pathway. In many signalling cascades, enzymes called kinases are activated. These enzymes add phosphate groups to other proteins or target molecules, changing the activity of their targets. If a kinase phosphorylates other kinases, a phosphorylation cascade may occur, in which multiple kinases are activated (and deactivated) to move the cell towards a response (Figure 5.34). In other types of signalling cascades, proteins called transcription factors are activated or inactivated. Transcription factors enter the nucleus and change gene expression by modifying mRNA synthesis of particular genes, which may be part of signal transduction or the cellular response.

Step 3: Cellular Response

The cellular response following signalling pathways is extremely varied and depend on the type of cell involved as well as the external and internal conditions. Many signalling pathways alter gene expression within the cell (Figure 5.35A), causing changes to the abundance of target proteins that will alter cellular structure or activity. For example, humans can produce substances called growth factors. When these growth factors bind to receptors on appropriate target cells, they can cause those cells to increase gene expression of c-Myc, a protein that helps regulate the cell cycle (cell division). Signalling pathways can also alter the activity of existing proteins in the cell, including enzymes in metabolic pathways (Figure 5.35B). For example, when the human hormone epinephrine binds to muscle cell receptors, it can trigger the de-activation of enzymes such as glycogen synthase, preventing the synthesis of new glycogen.

Step 4: Termination of the Signal Cascade

It is important for cells to be able to shut down the signal cascade once the cellular response has occurred. One method of stopping a specific signal is to degrade the ligand or remove it so that it can no longer access its receptor. Meanwhile, inside the cell, many different enzymes reverse the cellular modifications that result from signalling cascades. For example, phosphatases are enzymes that remove the phosphate group attached to proteins by kinases in a process called dephosphorylation (Figure 5.34). Second messengers can be degraded or removed from the cytosol. For example, cyclic AMP (cAMP) – a common second messenger – is degraded into AMP by the enzyme phosphodiesterase.

Types of Receptors

Cellular receptors are protein molecules found either on the surface of a target cell (cell surface receptors) or on the inside (internal receptors). These receptors allow ligands to bind to receptor active sites if they are the correct shape, or these receptors may change shape in response to other environmental signals such as temperature or pH. For simplicity, we will focus the following on receptors that bind ligands. The cell signalling pathway can look quite different depending on receptor location.

Internal Receptors

Internal receptors, also known as intracellular or cytoplasmic receptors, are found in the cytoplasm of the cell and respond to hydrophobic ligand molecules that can travel across the plasma membrane. Many of these receptor proteins double as transcription factors: proteins that can bind to DNA and alter gene expression. When the ligand binds to internal receptor that is also a transcription factor, a conformational change is triggered that exposes a DNA-binding site on the receptor. The ligand-receptor complex moves into the nucleus, then binds to specific regulatory regions of the chromosomal DNA and promotes or inhibits the initiation of transcription for specific genes. (Figure 5.36). Internal receptors can directly influence gene expression without having to pass the signal on to other receptors or messengers, skipping a lot of the steps involved in signal transduction discussed above. An example of an internal receptor in mammalian cells is the androgen receptor, which binds to testosterone (the ligand) and other hormones with a similar structure, stimulating changes in gene expression in a range of cell types.

Cell-Surface Receptors

Cell-surface receptors, also known as transmembrane receptors, are membrane-anchored (integral) proteins that bind to external ligand molecules. This type of receptor spans the plasma membrane and performs signal transduction, through which an extracellular signal is converted into an intracellular signal. Ligands that interact with cell-surface receptors are typically unable to pass through the cell membrane on their own, and the cell-surface receptor ensures they do not have to enter the cell that they affect.

Each cell-surface receptor has three main components: an external ligand-binding domain called the extracellular domain, a hydrophobic membrane-spanning region called a transmembrane domain, and an intracellular domain inside the cell. There are three general categories of cell-surface receptors: ion channel-linked receptors, enzyme-linked receptors, and G-protein-linked receptors.

Ion channel-linked receptors are transmembrane proteins that function both as receptors and ion channels. When a ligand binds the extracellular receptor portion of the protein, this causes the ion channel to open or close, changing whether specific ions can cross the membrane (Figure 5.37). The movement of ions into or out of the cell can elicit various cellular responses, especially in cells like neurons and muscles that use electrical (ion) signalling. For example, many neurotransmitters (chemicals that control activation and deactivation of neurons in animals) bind to ion channel-linked receptors.

Enzyme-linked receptors are transmembrane protein receptors with enzyme-associated intracellular domains. In some cases, the intracellular domain of the receptor itself is an enzyme. Other enzyme-linked receptors have a small intracellular domain that interacts directly with an enzyme. When a ligand binds to the extracellular domain of the enzyme-linked receptor, a signal is transferred through the membrane, activating the enzyme (Figure 5.38). Activation of the enzyme sets off a chain of events within the cell that eventually leads to a response. For example, insulin (a hormone that controls our blood sugar) binds to an enzyme-linked receptor in some cell types, activating a series of changes that results in more glucose transporters being embedded in the plasma membrane.

G-protein-linked receptors (GPCR) are transmembrane proteins that bind a ligand (extracellularly) and activate a peripheral membrane protein (intracellularly) called a G-protein (Figure 5.39). G proteins get their name from their ability to bind GTP and GDP (guanosine tri- and diphosphate, respectively). Cell signalling using G-protein-linked receptors occurs as a cyclic series of events (Figure 5.39). Before the ligand binds, the G-protein is inactive, and is bound to GDP. When a ligand binds the GPCR, this reveals a site on the receptor to which the G-protein can bind. Once the G-protein binds to the receptor, the resulting change in shape activates the G-protein, which releases GDP and picks up GTP. The subunits of the G-protein then split into the α subunit and the βγ subunit. One or both G-protein fragments may be able to activate other proteins as a result, facilitating signal transduction. The GTP on the active α subunit of the G-protein is then hydrolyzed to GDP and the βγ subunit is deactivated. The subunits reassociate to form the inactive G-protein and the cycle begins anew. Epinephrine (a hormone that helps coordinate our “fight or flight” response) is an example of a ligand that binds to GPCRs, which can cause a range of intracellular changes depending on the cell type.

Signalling Molecules – Ligands

Many types of signalling require a ligand. However, the types of molecules that serve as ligands are incredibly varied and range from small proteins to small ions like calcium (Ca²⁺). The properties of a ligand usually determine whether they interact with an intracellular or a cell-surface receptors. As we discussed in Chapter 5.2, only hydrophobic molecules (and a few very small hydrophilic molecules) are good at crossing membranes via simple diffusion, and many signalling molecules cannot enter our cells because they are large, hydrophilic, charged, or some combination of those!

Small Hydrophobic Ligands

Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors. Important members of this class of ligands are the steroid hormones. Steroids are lipids that have a hydrocarbon skeleton with four fused rings, and all steroid hormones are derived from cholesterol (Figure 5.40). Different steroids have different functional groups attached to the carbon skeleton. In humans, some examples of steroid hormones include the stress hormone, cortisol, and the sex hormones estradiol and testosterone (Figure 5.40).

Large or Hydrophilic Ligands

Water-soluble ligands are polar and, therefore, cannot pass through the plasma membrane unaided; sometimes, they are too large to pass through the membrane at all. Instead, most water-soluble ligands, such as epinephrine (adrenaline), bind to the extracellular domain of cell-surface receptors (Figure 5.38). This group of ligands is quite diverse and includes small molecules, peptides, and proteins (Figure 5.41). A good example of a hydrophilic ligand is insulin, a peptide hormone used by many animals to regulate blood sugar. Insulin binds to extracellular receptors to trigger an uptake of glucose, amino acids and fatty acids into liver cells, skeletal muscle cells and adipose cells. Failure for this process to work can result in diabetes, showing just how important signalling molecules are and their responsibility to maintain homeostasis.

Other Ligands

Another ligand which is both hydrophilic and hydrophobic is the gas nitric oxide (NO). In humans, it diffuses directly across the plasma membrane, and one of its roles is to interact with receptors in smooth muscle to induce relaxation of the tissue. NO has a very short half-life and, therefore, only functions over short distances. Nitroglycerin, a treatment for heart disease, acts by triggering the release of NO, which causes blood vessels to dilate, thus restoring blood flow to the heart. NO has become better known recently because the pathway that it affects is targeted by prescription medications for erectile dysfunction, such as Viagra, which aids in proper dilated blood vessels allowing erection.

Other signalling molecules besides ligands include viruses and bacteria. Viruses can bind to and infect host cells, while bacteria can bind to immune cell receptors and induce an immune response.

Second Messengers

Second messengers are molecules that carry a message and amplify it but are different from the ligand that activates the receptor. Cyclic AMP (cAMP) is a second messenger that is produced in the cytosol from ATP in a reaction catalyzed by the enzyme adenylyl cyclase (also called adenylate cyclase; Figure 5.42). Lots of cAMP can be produced in response to a single ligand binding to a single receptor, for example when epinephrine binds to an appropriate GPCR. In some cells, cAMP binds to and activates an enzyme called cAMP-dependent kinase (Protein kinase A). Protein kinase A activates many target proteins via phosphorylation (Figure 5.43).

Other second messengers that are used in many signalling pathways include inositol triphosphate (IP₃) and diacylglycerol (DAG). These two molecules are produced by cleavage of a particular membrane phosopholipid (PIP₂, phosphatidyl inositol bisphosphate) into IP₃ (inositol with three phosphate groups attached) and DAG (glycerol with two fatty acids and no phosphate headgroup), a reaction catalyzed by the enzyme phospholipase C (Figure 5.44). DAG remains in the membrane, while IP₃ enters the cytosol. DAG acts as a second messenger by activating an enzyme called protein kinase C. IP₃ binds to receptors on the endoplasmic reticulum to trigger the release of calcium ions into the cytosol, which can stimulate further signal transduction (Figure 5.45). Calcium ions (Ca²⁺) themselves can be considered a second messenger; usually they are at a low concentration in the cytosol, so an increase in cytosolic Ca²⁺ is a good signal that something has changed in a cell’s environment. Insulin is an example of a ligand that stimulates the use of IP₃ and Ca²⁺ as second messengers.

Forms of Signalling

Signalling is important for both unicellular and multicellular organisms. However, we’ll focus this section on the types of signalling that occur between cells within a multicellular organism like yourself! There are four categories of chemical signalling found in multicellular organisms: autocrine signalling, direct signalling, paracrine signalling, and endocrine signalling (Figure 5.46). The primary difference between each category of signalling is the distance that the signal must travel to reach the target cell. The signals transmitted to the target cells are hormones and other chemical messengers, which are produced or stored in the signal cell.

Autocrine Signalling

Autocrine signals are unique because the signal cell also becomes the target cell (Figure 5.46A). This means that the ligand produced by the signalling cells binds to the same cell. This type of signalling often occurs during the early development in animals to ensure correct tissue function and development. Autocrine signalling also regulates pain sensation and inflammatory responses. If an autocrine cell is infected with a virus, the cell can signal itself to undergo programmed cell death, killing the virus. In some cases, neighbouring cells of the same type are also influenced by the released ligand.

Endocrine Signalling

Signals from distant cells are called endocrine signals, and they originate from endocrine cells (Figure 5.46B). In animal bodies, most endocrine cells are found in endocrine glands, such as the thyroid gland, hypothalamus, and pituitary gland. Many of the examples discussed earlier in the chapter (testosterone, insulin, epinephrine) are hormones. These types of signals usually produce a slower response but have a longer-lasting effect. Endocrine signalling is unique from the other types of cell signalling because the ligands released are hormones. Hormones are signalling molecules that are produced in one part of the body but affect other body regions some distance away. Hormones travel large distances between endocrine cells and their target cells via the bloodstream or circulatory fluid, which is a relatively slow way to move throughout the body. Because of their form of transport, hormones become diluted and are present in low concentrations when they act on their target cells. This is different from paracrine signalling, in which local concentrations of ligands can be very high.

Direct Signalling

Direct signalling occurs by movement of signalling molecules directly from the cytosol of one cell to the other (Figure 5.46C). These signals can pass through gap junctions or plasmodesmata, which are found in animals and plants, respectively (Chapter 4). These fluid-filled channels allow small signalling molecules (called intracellular mediators) to diffuse between the two cells. Small molecules or ions, such as calcium ions (Ca²⁺), can move between cells, but large molecules like proteins and DNA cannot fit through the channels. The specificity of the channels ensures that the cells remain independent but can quickly and easily transmit signals. The transfer of signalling molecules communicates the current state of the cell that is directly next to the target cell; this allows a group of cells to coordinate their response to a signal that only one of them may have received. Direct signalling is important in, for example, contraction of your heart muscle cells.

Paracrine Signalling

Signals that act between cells that are close together are called paracrine signals (Figure 5.46D). Paracrine signals move by diffusion through the extracellular matrix. These types of signals usually elicit quick responses, lasting only a short period of time. To keep the response localized, paracrine ligand molecules are normally quickly degraded by enzymes or removed by neighbouring cells. One example of paracrine signalling from animals is the release of neurotransmitters from one neuron that bind to receptors on the adjacent neurons.