C2.1 Chemical signalling

Cells in a multicellular organism, and even single-celled organisms, communicate by chemical signals. One cell releases a signalling chemical; another cell, the target cell, detects it and responds.

Detection depends on receptors: proteins with a binding site for one particular signalling chemical. The signalling chemical is called the ligand (a general term for any molecule that binds to a specific site on another molecule).

• The binding site has a shape and chemical properties (charges, polarity, hydrophobic patches) that are complementary to the ligand, so only that ligand, or very similar molecules, bind. This specificity is like that of an enzyme’s active site, but the ligand is not changed by binding.
• Binding is by weak, non-covalent interactions, so it is reversible. The proportion of receptors with a ligand bound depends on the ligand’s concentration, so the size of the response can reflect how much signal there is.
• Binding causes a conformational change (change in shape) in the receptor. This is what passes the signal on.

A cell responds only to signals for which it has receptors. Every cell is exposed to many signals at once, but different cell types make different sets of receptors, so the same hormone can affect liver cells and have no effect at all on skin cells.

Bacteria also signal to one another. In quorum sensing, a population of bacteria uses a signalling chemical to sense its own population density and switches certain genes on only when the population is dense enough for them to be useful.

Bioluminescence in Vibrio fischeri

Vibrio fischeri is a marine bacterium that can produce light (bioluminescence) using the enzyme luciferase.

1. Every cell makes a signalling chemical called an autoinducer at a low rate, using an enzyme called LuxI. The autoinducer is a small molecule that diffuses freely through the cell membrane, in and out of cells.
2. When cells are few and far apart, as in open sea water, the autoinducer diffuses away and its concentration stays low.
3. In a dense population the autoinducer accumulates. Inside each cell it binds to a receptor protein, LuxR, in the cytoplasm.
4. Above a threshold concentration, the LuxR–autoinducer complex binds to a specific DNA sequence and promotes transcription of the lux genes, which code for luciferase and the enzymes that make its substrate. The cells emit blue-green light.
5. The lux genes include the gene for LuxI, so induced cells make more autoinducer, which reinforces the switch (positive feedback).

Why light only at high density?

Light from a single cell is too faint to have any effect, so making luciferase at low density would waste energy and materials. V. fischeri lives at very high density in the light organ of the Hawaiian bobtail squid (Euprymna scolopes). The squid supplies the bacteria with nutrients; the bacteria’s light, shining down from the squid’s underside, matches the moonlight from above, so predators below do not see the squid’s shadow (counter-illumination). It is an example of interaction between cells (signals pass from cell to cell) and interdependence (no single cell can produce the effect).

Animals use many signalling chemicals, which can be grouped by what they do and how far they travel.

Some points that distinguish the categories:

• Hormones and neurotransmitters differ mainly in distance and duration (next sections). The same chemical can be both: noradrenaline is released by neurons at synapses and also secreted into the blood as a hormone.
• Cytokines are small proteins. Unlike hormones, most act locally. One cytokine can be made by several cell types and can have several effects.
• Calcium ions are unusual: an inorganic ion, mostly acting inside cells. Pumps keep the Ca²⁺ concentration in the cytoplasm very low (roughly ten-thousand times lower than outside), so opening Ca²⁺ channels causes a sudden rise that the cell detects as a signal.

Signalling chemicals belong to many different chemical groups.

Why so many different chemicals?

• Each signal must be distinguishable: a receptor can only respond to the right signal if each signal has a distinct shape and chemical properties.
• Signals with different properties suit different jobs. Hydrophilic peptides and most amines dissolve in blood and act at the cell surface, giving fast responses. Hydrophobic steroids cross membranes and change gene expression, giving slower but longer-lasting responses. A small gas such as nitric oxide diffuses in all directions through membranes, including back to the presynaptic cell, and is broken down within seconds.
• Signalling systems have evolved many times, and each has used molecules that were already available, such as amino acids and their derivatives, or cholesterol.

Signalling chemicals differ in how far they travel, which decides how many cells they affect and how quickly.

Neurotransmitters act locally. They are released from the end of a presynaptic neuron into a synaptic gap only about 20–40 nm wide, diffuse across it in a fraction of a millisecond, and bind to receptors on the membrane of the one postsynaptic cell. They are then rapidly broken down or reabsorbed, so each signal is brief and goes to a single, precise destination.

Hormones act at a distance. They are secreted by endocrine glands into the blood and carried all around the body, which takes from seconds to about a minute. Every cell with the right receptors responds, wherever it is: for example, LH secreted by the pituitary gland at the base of the brain acts on cells in the ovaries or testes. Hormones persist in the blood for minutes to hours, so their effects are widespread and longer-lasting.

Whether a signal can enter the cell depends on whether it can cross the hydrophobic core of the plasma membrane, and this decides where its receptor is.

Transmembrane receptors are integral proteins of the plasma membrane. Hydrophilic signals, such as peptide and protein hormones (insulin), amines (adrenaline) and most neurotransmitters, cannot pass through the bilayer, so they remain outside and bind to the receptor’s outer part.

• The parts of the receptor outside and inside the cell have hydrophilic amino acids on their surface, in contact with water.
• The part within the membrane has a band of hydrophobic amino acids, in contact with the fatty acid tails. This holds the receptor in the membrane.

Intracellular receptors are in the cytoplasm or nucleus. Their ligands are hydrophobic signals, mainly steroid hormones, which pass through the bilayer by simple diffusion and enter the cell.

• The receptor has hydrophilic amino acids over its surface, so it is dissolved in the cytoplasm or nucleoplasm and is not held in a membrane.
• Its binding site is lined with hydrophobic amino acids that fit the steroid.

Binding of a ligand to its receptor sets off a sequence of responses inside the cell, called a signal transduction pathway. “Transduction” means the signal is converted from one form to another: from a chemical outside the cell into a change in shape of the receptor, and then into a chain of interactions between molecules inside.

The pathways differ for the two kinds of receptor:

• Transmembrane receptors: binding changes the shape of the part of the receptor in the cytoplasm. This may open an ion channel (the acetylcholine receptor), activate a G protein (GPCRs), or activate an enzyme activity of the receptor itself (tyrosine kinase receptors). Often a second messenger such as cyclic AMP is made inside the cell, and protein kinases (enzymes that add phosphate groups to other proteins) pass the signal from enzyme to enzyme.
• Intracellular receptors: the hormone–receptor complex binds to DNA and changes which genes are transcribed.

Two features are common to many pathways:

• Amplification: each activated molecule activates many molecules of the next stage, so a few ligand molecules can produce a large response.
• Reversibility: the signal is switched off when the ligand leaves the receptor and the intermediates are inactivated, so the cell can respond to the next change.

Pathways are very diverse, because they have evolved many times rather than from a single origin. You need three in detail: the G protein pathway for adrenaline, the tyrosine kinase pathway for insulin and the intracellular pathway for steroid hormones. Step through all three in the 3D model below (switch models with the buttons above it).

Receptors for neurotransmitters are transmembrane proteins in the plasma membrane of postsynaptic neurons and muscle fibres. Many of them are themselves ion channels, so they can change the voltage across the membrane (the membrane potential) very quickly.

The example to learn is the acetylcholine receptor (the nicotinic type, at synapses between motor neurons and skeletal muscle fibres, and between many neurons):

1. Acetylcholine released by the presynaptic neuron diffuses across the synaptic gap and binds to binding sites on the receptor.
2. Binding causes a conformational change that opens a channel through the middle of the receptor.
3. Positively charged ions, mainly sodium ions (Na⁺), diffuse into the cell through the channel, down their concentration and electrical gradients.
4. The inside of the cell becomes less negative (depolarization). If the change is large enough it triggers an action potential in a neuron, or contraction of a muscle fibre.
5. Acetylcholine is quickly broken down in the synaptic gap by the enzyme acetylcholinesterase, so the channel closes and the signal ends.

No second messenger is needed, which is why this kind of signalling is so fast. See B2.1 Membranes and membrane transport for a simulation of this gated channel and C2.2 Neural signalling for how action potentials and synapses work.

G protein-coupled receptors (GPCRs) are transmembrane receptors that pass a signal into the cell through a second protein, a G protein, on the cytoplasmic side of the membrane. A GPCR is a single polypeptide that crosses the membrane seven times, with its ligand-binding site on the outside.

The G protein has three subunits, α, β and γ. In the resting state it is attached to the inside of the receptor, and its α subunit carries a molecule of GDP (guanosine diphosphate), which keeps it inactive.

1. A ligand binds to the receptor’s binding site outside the cell.
2. The receptor changes shape, which makes the α subunit release GDP and bind GTP (guanosine triphosphate) in its place.
3. With GTP bound, the G protein is active. The α subunit separates from β and γ and from the receptor.
4. The activated subunits interact with other proteins in the membrane or cytoplasm (effectors), such as enzymes or ion channels, starting the cell’s response.
5. The α subunit slowly hydrolyses its GTP to GDP, which switches it off; it then rejoins β and γ.

One activated receptor can activate many G proteins before the ligand leaves it, so the signal is amplified at the first step.

Many receptors, one mechanism

Humans have about 800 different GPCRs, encoded by about 4% of our genes. They detect a huge range of signals: hormones such as adrenaline and glucagon, many neurotransmitters, odour molecules in the nose, and even light (rhodopsin in the retina). Because they are so widespread, roughly a third of medical drugs act on GPCRs.

Adrenaline is secreted by the adrenal glands in “fight or flight” situations. One of its effects is to make the liver release glucose into the blood within seconds. Its receptors are GPCRs in the plasma membrane of target cells such as liver cells.

1. Adrenaline binds to the receptor on the outside of the liver cell; it does not enter the cell.
2. The receptor activates a G protein (GDP is replaced by GTP on the α subunit).
3. The α subunit activates adenylyl cyclase, an enzyme in the plasma membrane.
4. Adenylyl cyclase converts ATP into cyclic AMP (cAMP). Each enzyme makes many cAMP molecules.
5. cAMP is the second messenger (adrenaline is the first). It spreads through the cytoplasm and activates protein kinase A.
6. Protein kinase A starts a cascade of phosphorylation: it activates phosphorylase kinase, which activates glycogen phosphorylase. Each enzyme activates many molecules of the next, so the signal is greatly amplified.
7. Glycogen phosphorylase breaks down glycogen, and the glucose produced is released into the blood.

The response stops when adrenaline concentration falls: the G protein hydrolyses its GTP and cAMP is broken down by phosphodiesterase.

A kinase is an enzyme that transfers a phosphate group from ATP to another molecule. Phosphorylation changes the shape of a protein, often switching it on. A tyrosine kinase adds phosphate to the amino acid tyrosine in proteins.

Some transmembrane receptors are themselves tyrosine kinases. The example is the receptor for the protein hormone insulin, which is secreted by β cells of the pancreas when the blood glucose concentration is high.

1. The insulin receptor is made of two linked halves (a dimer). Each half crosses the membrane and has a tyrosine kinase domain in the cytoplasm.
2. Insulin binds to the part of the receptor outside the cell. It does not enter the cell.
3. Binding causes a conformational change that brings the two kinase domains together, and each phosphorylates tyrosines on the other half (autophosphorylation).
4. The phosphorylated tyrosines act as docking sites for relay proteins, which are phosphorylated in turn, starting a sequence of reactions inside the cell.
5. The sequence ends with vesicles containing glucose transporters (GLUT4) moving to the plasma membrane and fusing with it.
6. With more transporters in the membrane, glucose enters the cell by facilitated diffusion, and the blood glucose concentration falls.

This happens mainly in muscle and fat (adipose) cells, which store most of their GLUT4 inside the cell until insulin arrives. Use the Tyrosine kinase receptor model in the 3D viewer above to see each step.

Steroid hormones, including oestradiol, progesterone and testosterone, are made from cholesterol. They are hydrophobic, so they diffuse straight through the plasma membrane and act through intracellular receptors.

1. The hormone diffuses through the phospholipid bilayer into the cytoplasm.
2. It binds to a specific binding site on its receptor, activating the receptor by changing its shape. Some steroid receptors wait in the cytoplasm (as for testosterone) and others are mostly already in the nucleus (as for oestradiol).
3. The hormone–receptor complex enters the nucleus through a nuclear pore.
4. The activated receptor binds to specific DNA sequences (hormone response elements) near particular genes. The receptor is acting as a transcription factor.
5. This promotes transcription of those genes, so more of their mRNA and protein are made. (Some genes are repressed instead.)

Only genes with the right DNA sequence nearby are affected, and only cells with the receptor respond. Because the response depends on making new mRNA and protein, it takes longer (minutes to hours) than responses through membrane receptors, but it lasts longer. See the Intracellular receptor model in the 3D viewer above, and D2.2 Gene expression for how gene expression is regulated.

Oestradiol and progesterone are steroid hormones secreted by the ovaries (and by the corpus luteum and, in pregnancy, the placenta). Both act through intracellular receptors and change gene expression in their target cells.

Oestradiol and GnRH

For most of the menstrual cycle, oestradiol at moderate levels reduces GnRH secretion (negative feedback). Late in the follicular phase, when developing follicles secrete high levels of oestradiol for about two days, its effect reverses: it stimulates GnRH secretion and makes the pituitary more sensitive to GnRH. This is the positive feedback that causes the LH surge and ovulation (next section).

Progesterone and the endometrium

After ovulation the corpus luteum secretes progesterone. In endometrium cells it binds to progesterone receptors and changes gene expression so that the endometrium is maintained and prepared for implantation. If no pregnancy occurs, the corpus luteum breaks down, progesterone falls and the endometrium is shed (menstruation). See D3.1 Reproduction for the whole menstrual cycle.

Signalling pathways are regulated by feedback: the amount of the pathway’s end product affects how much more is produced, usually by acting on an early step.

Negative feedback: the product inhibits its own production. More product leads to less production, and less product leads to more, so a change in either direction is reversed and the level stays near a set point.

• Example: thyroxine. The hypothalamus secretes TRH, which stimulates the anterior pituitary to secrete TSH, which stimulates the thyroid gland to secrete thyroxine. High thyroxine inhibits secretion of TRH and TSH, so thyroxine falls; low thyroxine removes the inhibition, so it rises again.

Positive feedback: the product stimulates its own production. More product leads to more production, so a change is amplified and the level rises rapidly to a peak. It needs something else to end it.

• Example: the LH surge. Developing follicles secrete rising amounts of oestradiol, which stimulates GnRH and LH secretion; LH stimulates the follicles to secrete even more oestradiol. LH rises steeply to a peak that triggers ovulation, after which oestradiol falls and the loop ends.

Feedback also works within single cells: in quorum sensing, the autoinducer switches on the gene for its own synthase (positive feedback). Negative feedback is far more common; see D3.3 Homeostasis for homeostasis and C3.1 Integration of body systems for how the nervous and endocrine systems are integrated.