Introduction

Cell signalling, also called signal transduction, describes the ability of cells to respond to stimuli from their environment. Examples include healing a wound after injury, activating the immune system in response to pathogens, and changing gene expression during different developmental stages. Communication between cells is called intercellular signalling, and communication within a cell is called intracellular signalling. An easy way to remember the distinction is by understanding the Latin origin of the prefixes: inter- means “between” (for example, intersecting lines are those that cross each other), and intra- means “inside” (like intravenous).

Unit 4 will examine the basic principles of how cells communicate with one another will be examined. First, it will look at how cell-cell signalling works, and then it will consider different kinds of short- and long-range signalling that happen in the human body.

Unit 4, Topic 1—To Do List	Suggested Average Time to Complete (min.)
Read, make summary notes, and complete the self-check questions from Unit 4, Topic 1 of TRU Cell and Molecular Biology.	60
Complete Learning Activity: Overview of cell signalling.	10
Complete Learning Activity: How neurons communicate.	5
Complete Learning Activity: Categories of cell signalling.	5
Complete Learning Activity: Membrane receptors.	15
Complete Learning Activity: Ligand-gated ion channels.	15
Complete Learning Activity: G-Protein-coupled receptors (GPCRs).	30
Complete Learning Activity: Enzyme-linked receptors.	10
Complete Learning Activity: Summary — Receptors and secondary messenger systems.	5
Complete Learning Activity: Summary — Introduction to Cell Signalling.	30
Complete Learning Activity: Mechanisms of hormone action.	10

Overview of Signalling — Molecules and Receptors

Chemical signals are released into the extracellular space by signalling cells in the form of small, usually volatile or soluble molecules called ligands. A ligand is a molecule that binds another specific molecule, in some cases, delivering a signal in the process (Figure 1). Ligands can thus be thought of as chemical messages. Ligands interact with proteins in target cells, which are cells affected by chemical signals; these proteins are called receptors. Ligands and receptors exist in several varieties; however, a specific receptor typically only binds to a specific ligand. Not all cells can “hear” a particular chemical message. Detecting a signal (being a target cell) requires that cell to have the right receptor for that signal.

 

When a signalling molecule binds to its receptor, it alters the shape or activity of the receptor, triggering a change inside the cell (Figure 2). The message carried by a ligand often relays through a chain of chemical messengers inside the cell. Ultimately, it leads to a change in the cell, such as an alteration in the activity of a gene or even the induction of a whole process, such as cell division or protein synthesis. Thus, the original intercellular (between-cells) signal converts into an intracellular (within-cell) signal that triggers a response.

Forms of Cell Signalling

Cell signalling can divide into the following four major categories depending on how far the signalling molecule is from the receptor and where the receptor is (Figure 3):

• Long-range signalling (endocrine)
• Short-range signalling (paracrine)
• Self-activation (autocrine)
• Direct activation (contact-dependent, also called juxtacrine)

Endocrine Signalling

When cells need to transmit signals over long distances, they often use the circulatory system as a distribution network for the messages they send. In long-distance endocrine signalling, specialized cells produce signals and release them into the bloodstream, carrying them to target cells in distant parts of the body (Figure 4). Signals from distant cells are called endocrine signals, and they originate from endocrine cells. In the body, many endocrine cells are in endocrine glands, such as the thyroid gland, hypothalamus, pituitary gland, as well as the gonads and pancreas. Endocrine signals usually produce a slower response but have a longer-lasting effect. The ligands released in endocrine signalling are called hormones, signalling molecules produced in one part of the body but affecting other body regions some distance away. Each endocrine gland releases one or more types of hormones, many of which are master regulators of development and physiology.

 

For example, the pituitary gland releases growth hormone (GH), which promotes growth, particularly of the skeleton and cartilage. Like most hormones, GH affects many different types of cells throughout the body. However, cartilage cells provide one example of how GH functions: it binds to receptors on the surface of these cells and encourages them to divide.

Hormones travel large distances between endocrine cells and their target cells via the bloodstream, 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.

Paracrine Signalling

Signals acting locally between cells sitting close together are known as paracrine signals (Figure 5). Paracrine signals move by diffusion through the extracellular matrix. These types of signals usually elicit quick responses that last only a short period. The body keeps this response localized by using enzymes to degrade paracrine ligand molecules or using neighbouring cells to remove them. Removing the signals re-establishes the concentration gradient for the signal, allowing for quick diffusion through the intracellular space if released again.

 

One example of paracrine signalling is the transfer of signals across synapses between nerve cells. A nerve cell consists of a cell body, several short, branched extensions called dendrites that receive stimuli, and a long extension called an axon, which transmits signals to other nerve cells or muscle cells. The junction between nerve cells where signal transmission occurs is called a synapse. A synaptic signal is a chemical signal that travels between nerve cells. Fast-moving electrical impulses propagate the signals within the nerve cells. When the sending neuron fires, an electrical impulse quickly moves through the cell, travelling down a long, fibre-like extension called an axon. When the impulse reaches the synapse, it triggers the release of chemical ligands called neurotransmitters from the presynaptic cell (the cell emitting the signal), which quickly cross the small gap between the neurons. The neurotransmitters are transported across very small distances (20–40 nm) between nerve cells, called chemical synapses (Figure 6). The small distance between nerve cells allows the signal to travel quickly; this enables an immediate response, such as, “Take your hand off the stove!”

When the neurotransmitters arrive at the receiving cell, they bind to receptors and cause a chemical change inside the cell (often opening ion channels and changing the electrical potential across the membrane). The neurotransmitters released into the chemical synapse degrade quickly or get reabsorbed by the presynaptic cell so that the recipient nerve cell can recover quickly and prepare to respond rapidly to the next synaptic signal (Figure 6).

 

Autocrine Signalling

Signalling cells produce autocrine signals and can also bind to the released ligand (Figure 5). This means the signalling cell and the target cell can be the same or a similar cell (the prefix auto- means self, a reminder that the signalling cell sends a signal to itself). This type of signalling often occurs during the early development of an organism to ensure cells develop into the correct tissues and take on the proper function. Autocrine signalling also regulates pain sensation and inflammatory responses. Further, if a cell becomes infected with a virus, it can signal itself to undergo programmed cell death, killing the virus in the process. In some cases, the released ligand also influences neighbouring cells of the same type. In embryological development, this process of stimulating a group of neighbouring cells may help to direct the differentiation of identical cells into the same cell type, thus ensuring the proper developmental outcome.

Direct Signalling Across Gap Junctions

Gap junctions in animals and plasmodesmata in plants are connections between the plasma membranes of neighbouring cells. These water-filled channels allow small signalling molecules, called intracellular mediators, to diffuse between the two cells (Figure 7). Small molecules, 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. In plants, plasmodesmata are ubiquitous, making the entire plant into a giant communication network.

 

In another form of direct signalling, two cells may bind to one another because they carry complementary proteins on their surfaces. When the proteins bind to one another, this interaction changes the shape of one or both proteins, transmitting a signal. This kind of signalling is especially important in the immune system, where immune cells use cell-surface markers to recognise “self” cells (the body’s own cells) and cells infected by pathogens (Figure 8).

 

Types of Receptors

Receptors are protein molecules in the target cell or on its surface that bind ligands. There are two types of receptors: internal receptors and cell-surface receptors.

Internal Receptors

Internal receptors, also known as intracellular or cytoplasmic receptors, are found in the cytoplasm or nucleus of the cell and respond to small hydrophobic ligand molecules that can travel across the plasma membrane. The hydrophobicity of these ligands allows them to cross the plasma membrane. For example, the primary receptors for hydrophobic steroid hormones, such as the sex hormones estradiol (an estrogen) and testosterone, are intracellular.

Once inside the cell, many of these molecules bind to proteins that regulate mRNA synthesis (transcription) to mediate gene expression. Gene expression is the cellular process of transforming the information in a cell’s DNA into a sequence of amino acids, which ultimately form a protein. Intracellular receptors often function as a transcriptional activator by binding specific DNA elements, altering the transcription of downstream genes. Intracellular receptors commonly have three domains: ligand-binding domain, DNA-binding domain, and transactivation domain. When the ligand binds to the internal receptor, it triggers a conformational change that exposes a DNA-binding site on the protein. The ligand-receptor complex moves into the nucleus, then binds to specific regulatory regions of the chromosomal DNA and promotes the initiation of transcription (Figure 9). Transcription is the process of copying the information in a cell’s DNA into a special form of RNA called messenger RNA (mRNA); the cell uses information in the mRNA (which moves out into the cytoplasm and associates with ribosomes) to link specific amino acids in the correct order, producing a protein. Internal receptors can directly influence gene expression without passing the signal on to other receptors or messengers. The signal terminates when the hormone concentration lowers.

 

Cortisol is an example of a hormone that binds an intracellular receptor. The adrenal cortex releases cortisol, which diffuses into the bloodstream attached to serum albumin and steroid hormone-binding globulin. After diffusing through the plasma membrane, it binds to the cortisol receptor (intracellular receptor) in the cytosol and forms a homodimer, exposing a nuclear localization signal (NLS). Exposure of the NLS targets the complex to the nucleus.

Cell-Surface Receptors

Cell-surface receptors, also known as transmembrane receptors, are cell surface, membrane-anchored (integral) proteins that bind to external ligand molecules. This receptor type spans the plasma membrane and performs signal transduction, where an extracellular signal converts into an intracellular signal. Ligands that interact with cell-surface receptors do not have to enter the cell that they affect. Cell-surface receptors are also known as cell-specific proteins or markers because they are specific to individual cell types.

Receptors, even at high density, represent only a minute fraction of the cell’s surface area and, therefore, an even smaller fraction of the volume of the cell. Therefore, activating a receptor must be amplified to initiate cellular activities (e.g., locomotion, growth, cell cycle progression). Thus, one of the first things a receptor does upon activation or ligand binding is to initiate an intracellular signalling cascade (Figure 10).  The molecules within an intracellular signalling cascade will produce small intracellular molecules that amplify the cellular response to the primary signalling molecules. These molecules are termed secondary messengers (Figure 10).

 

Because cell-surface receptor proteins are fundamental to normal cell functioning, it should be no surprise that a malfunction in any of these proteins could have severe consequences. Errors in the protein structures of specific receptor molecules have played a role in hypertension (high blood pressure), asthma, heart disease, and cancer.

Each cell-surface receptor has three main components: an extracellular ligand-binding domain (“outside of cell”), a hydrophobic membrane-spanning region called a transmembrane domain, and an intracellular domain inside the cell. The size and extent of each of these domains vary widely, depending on the type of receptor. The hydrophobic region may consist of multiple stretches of amino acids that crisscross the membrane (Figure 11).

Cell-surface receptors are involved in most signalling in multicellular organisms. There are three general categories of cell-surface receptors: ligand-gated ion channels, G-protein-coupled receptors, and enzyme-linked receptors.

Ligand-Gated Ion Channels

Ligand-gated ion channels act as receptors that bind a ligand, which opens the channel through the membrane and allows specific ions to pass through. This type of cell-surface receptor has an extensive membrane-spanning region to form a channel. In order to interact with the double layer of phospholipid fatty acid tails that form the centre of the plasma membrane, many amino acids in the membrane-spanning region are hydrophobic. Conversely, the amino acids lining the inside of the channel are hydrophilic to allow for the passage of water or ions. When a ligand binds to the extracellular region of the channel, there is a conformational change in the protein’s structure that allows ions such as sodium, calcium, magnesium, and hydrogen to pass through (Figure 12).

When a ligand binds to the extracellular region of the channel, the protein’s structure changes in such a way that ions of a particular type, such as Ca²⁺, can pass through. In some cases, the reverse is true: the channel is usually open, and ligand binding causes it to close. Changes in ion levels inside the cell can change the activity of other molecules, such as ion-binding enzymes and voltage-sensitive channels, to produce a response. Neurons, or nerve cells, have ligand-gated channels bound by neurotransmitters.

G-Protein-Coupled Receptors

G-protein-coupled receptors (GPCRs) bind a ligand and activate a membrane protein called a G-protein. G-proteins are GTPases that hydrolyze a bound GTP to GDP, where the GDP molecule remains bound to the GTPase. Although the GTPase can hydrolyze GTP spontaneously, the GTPase-activating protein, GAP, greatly speeds up the hydrolysis rate. When GDP binds, the protein becomes inactive. Cycling the system back to GTP and activating the protein does not involve re-phosphorylating the GDP; instead, the GDP must be removed and exchanged for a new GTP. An accessory protein, the guanine nucleotide exchange factor (GEF), facilitates this exchange (Figure 13).

The activated G-protein then interacts with either an ion channel or an enzyme in the membrane (Figure 14). All GPCRs have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site. GPCRs are diverse and bind many different types of ligands. One particularly interesting class of GPCRs is the odorant (scent) receptors. There are about 800 of them in humans, and each binds its own “scent molecule” — such as a particular chemical in perfume or a specific compound released by rotting fish — and causes a signal to be sent to the brain, making people smell a smell.

Cell signalling using GPCRs occurs as a cyclic series of events (Figure 14). Before the ligand binds, the inactive G-protein can bind to a newly revealed site on the receptor specific for its binding. 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 of these G-protein fragments may be able to activate other proteins as a result. After a while, the GTP on the active α subunit of the G-protein undergoes hydrolysis to GDP, and the βγ subunit is deactivated. The subunits reassociate to form the inactive G-protein, and the cycle begins anew.

 

GPCRs have been extensively studied, so much has been learned about their roles in maintaining health. Bacteria pathogenic to humans can release poisons to interrupt specific GPCR functions, leading to illnesses like pertussis, botulism, and cholera. For example, in cholera (Figure 15),  the water-borne bacterium Vibrio cholerae produces a toxin, choleragen, that binds to cells lining the small intestine. The toxin then enters these intestinal cells, where it modifies a G-protein that controls the opening of a chloride channel and causes it to remain continuously active, resulting in large losses of fluids from the body and potentially fatal dehydration as a result.

 

Enzyme-Linked Receptors

Enzyme-linked receptors are cell-surface receptors with intracellular domains associated with an enzyme. In some cases, the receptor’s intracellular domain is an enzyme itself. The enzyme-linked receptors usually have large extracellular and intracellular domains, but the membrane-spanning region consists of a single alpha-helical region of the peptide strand. When a ligand binds to the extracellular domain, a signal is transferred through the membrane, activating the enzyme. Activation of the enzyme sets off a chain of events within the cell, eventually leading to a response.

One example of this type of enzyme-linked receptor is the tyrosine kinase family of receptors (Figure 16). A kinase is an enzyme that transfers phosphate groups from ATP to another protein. Kinases add a phosphate group to the hydroxyl group of serine, threonine, and/or tyrosine residues of the protein substrate. Various kinases have names related to the substrate they phosphorylate. Phosphorylation of serine and threonine residues often activates enzymes. Phosphorylation of tyrosine residues can either affect the activity of an enzyme or create a binding site that interacts with downstream components in the signalling cascade.

The tyrosine kinase receptor transfers phosphate groups to the amino acid tyrosine (tyrosine residues). First, signalling molecules bind to the extracellular domain of two nearby tyrosine kinase receptors. The two neighbouring receptors then bond together or dimerize. Phosphates are then added to tyrosine residues on the intracellular domain of the receptors (mutual phosphorylation). The phosphorylated residues can then transmit the signal to the next messenger within the cytoplasm.

Receptor tyrosine kinases are crucial to many signalling processes in humans. For instance, they bind to growth factors, signalling molecules that promote cell division and survival. Growth factors include platelet-derived growth factor, which participates in wound healing, and nerve growth factor, which certain types of neurons require a continual supply of to stay alive. Because of their role in growth factor signalling, receptor tyrosine kinases are essential in the body. However, their activity must stay balanced as overactive growth factor receptors are associated with some types of cancers.

Signalling Molecules

Produced by signalling cells and the subsequent binding to receptors in target cells, ligands act as chemical signals that travel to the target cells to coordinate responses. The types of molecules that serve as ligands  vary greatly and range from small proteins to small ions like calcium (Ca²⁺). Others are hydrophobic molecules, like steroids, and gases, like nitric oxide. This section looks at some examples of different types of ligands.

Ligands That Can Enter the Cell

Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors in the nucleus or cytoplasm. In the human body, some of the most crucial ligands of this type are the steroid hormones. Steroids are lipids with a hydrocarbon skeleton with four fused rings; different steroids have different functional groups attached to the carbon skeleton. Familiar steroid hormones include the female sex hormone, estradiol (a type of estrogen); the male sex hormone, testosterone; and cholesterol, a crucial structural component of biological membranes and a precursor of steroid hormones (Figure 17). Other hydrophobic hormones include thyroid hormones and vitamin D (synthesized in the skin using energy from light). Because they are hydrophobic, these hormones do not have trouble crossing the plasma membrane, but they must bind to carrier proteins to travel through the (watery) bloodstream.

Water-Soluble 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 bind to the extracellular domain of cell-surface receptors. This group of ligands is quite diverse and includes small molecules, peptides, and proteins.