Cell Biology: Chapter 12 — Cell signaling

12.1 The logic of a pathway #

12.1.1 Reception, transduction, response #

A signal is information, usually a molecule at a low concentration. The cell detects it with a receptor that has affinity and specificity, converts the detection into an intracellular change (transduction), and answers with a response: a change in enzyme activity, in the cytoskeleton, in gene expression, or in the decision to divide or to die. The same signal can mean different things to different cells because the cells express different receptors or different downstream wiring. Acetylcholine slows heart muscle and contracts skeletal muscle. The ligand is the same; the receptors and the effectors are not.

Pathways amplify. One occupied receptor can activate many G proteins, each of which can activate an enzyme that produces many second-messenger molecules, each of which can activate a protein kinase that phosphorylates many substrates. They also terminate. A signal that cannot be turned off is a tumour or a seizure. Phosphatases, phosphodiesterases, GTPase activity, receptor endocytosis, and inhibitory feedback exist so that the pathway follows the ligand and then stops.

12.1.2 Range #

Synaptic signalling is paracrine in mechanism and unique in speed and privacy: the neurotransmitter diffuses a few tens of nanometres across the cleft and is then removed or degraded.

12.2 G-protein-coupled receptors #

12.2.1 The receptor and the G protein #

GPCRs are the largest receptor family. Each has seven transmembrane helices. Ligand binding, on the outside or within the helical bundle, changes the conformation so that the cytosolic loops act as a guanine-nucleotide exchange factor for a heterotrimeric G protein.

The G protein has subunits α, β, and γ. In the resting state α holds GDP and the trimer is together. The activated receptor promotes release of GDP. GTP binds, α lets go of βγ, and both α-GTP and free βγ can signal. The α subunit is a slow GTPase. When it hydrolyses GTP it rebinds βγ and signalling stops. The slowness is the point: one exchange event gives a timed pulse of activity. Cholera toxin and pertussis toxin lock particular α subunits on or off by modifying this GTPase cycle, which is why they cause relentless signalling in the gut epithelium (cholera, Gαs locked on, massive cAMP, secretory diarrhoea) or block inhibitory signalling (pertussis).

12.2.2 Three effector systems #

Gαs and cAMP. Gαs-GTP stimulates adenylyl cyclase, which makes cyclic AMP from ATP. cAMP binds the regulatory subunits of protein kinase A and releases the catalytic subunits. PKA phosphorylates metabolic enzymes (glycogen breakdown in muscle and liver in response to adrenaline or glucagon) and transcription factors such as CREB. cAMP is destroyed by phosphodiesterases. Caffeine inhibits some of them, which is why it prolongs cAMP signals. Gαi inhibits adenylyl cyclase and therefore lowers cAMP.

Gαq, IP3, and DAG. Gαq activates phospholipase C-β, which cleaves the membrane lipid PIP₂ into two second messengers. IP3 is water-soluble and opens IP3-gated Ca2+ channels in the ER, so cytosolic Ca2+ rises. DAG stays in the membrane and, together with Ca2+, activates protein kinase C. This pathway controls secretion, smooth-muscle contraction, and many other short-term responses.

Ion channels. Some G proteins open or close channels directly, without a second-messenger enzyme. In the heart, parasympathetic acetylcholine uses a Gi-coupled receptor whose βγ subunit opens a K+ channel, hyperpolarises the pacemaker cell, and slows the heart. This is the fastest GPCR response because it skips the enzyme steps.

12.2.3 Calcium as a messenger #

Cytosolic Ca2+ is kept near 100 nM by pumps (Chapter 3). Opening a channel in the ER or the plasma membrane raises it locally toward micromolar. The rise is read by sensors, above all calmodulin, which changes shape when occupied by Ca2+ and then binds target enzymes such as CaM kinase and myosin-light-chain kinase. Ca2+ is used so widely that specificity comes from where and how briefly it rises, not from the ion itself. Mitochondria and the ER shape those local domains.

12.3 Receptor tyrosine kinases #

12.3.1 Dimerisation and autophosphorylation #

Most growth-factor receptors (EGF, PDGF, NGF, insulin) are receptor tyrosine kinases, or are tightly associated with one. For receptors such as the EGF receptor, ligand binding brings two kinase domains together. The insulin receptor is already a disulfide-linked dimer, and insulin activates it by a conformational change rather than by assembling the dimer from scratch. In either case the kinase domains phosphorylate each other on tyrosine residues. The phosphorylated tyrosines are docking sites for cytosolic proteins that carry SH2 or PTB domains. The receptor does not have to phosphorylate every downstream protein itself. It builds a local cluster of adaptors, and those carry the signal onward. Different receptors dock different sets, which is why insulin and EGF do not have identical effects.

Endocytosis of the activated receptor, followed by degradation in the lysosome or by recycling, limits the signal. A receptor that cannot be internalised signals for too long.

12.3.2 Ras and the MAP kinase cascade #

The pathway used by many growth factors to drive proliferation:

1. An adaptor (Grb2) binds a phosphotyrosine and brings the exchange factor Sos to the membrane.

2. Sos activates Ras, a small GTPase, by loading it with GTP. Ras is tethered to the membrane by a lipid anchor.

3. Ras-GTP recruits the kinase Raf (a MAP kinase kinase kinase).

4. Raf phosphorylates MEK (a MAP kinase kinase), which phosphorylates ERK (a MAP kinase).

5. ERK phosphorylates cytosolic targets and, after entering the nucleus, transcription factors that turn on immediate-early genes, including the cyclin D gene that links this chapter to Chapter 11.

The cascade is a coincidence detector and an amplifier. It also has negative feedback: ERK signalling induces phosphatases that later dampen the pathway.

Ras is a proto-oncogene. A point mutation that destroys its GTPase activity, so that it stays in the GTP form without any growth factor, drives inappropriate proliferation. Such mutations are common in human cancers. The normal gene is the proto-oncogene. The locked-on allele is the oncogene.

12.3.3 PI3K and Akt, the survival branch #

A second cluster at the activated receptor is phosphoinositide 3-kinase. It phosphorylates PIP₂ to PIP₃. PIP₃ recruits the kinase Akt (protein kinase B) to the membrane, where Akt is activated. Akt promotes survival by phosphorylating and thereby inhibiting pro-apoptotic proteins, and it promotes growth by activating mTOR signalling. PTEN, a phosphatase that converts PIP₃ back to PIP₂, opposes the pathway and is a tumour suppressor. Loss of PTEN is another frequent route to a cell that acts as if growth factors were always present.

12.4 Intracellular receptors and pathway interaction #

The study of Intracellular receptors and pathway interaction provides fundamental principles governing cellular architecture, molecular interactions, and physiological regulation.

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12.4.1 Steroid and thyroid hormones #

Steroid hormones, thyroid hormone, retinoic acid, and vitamin D are small and hydrophobic enough to cross the plasma membrane. Their receptors are intracellular, often already in the nucleus or waiting in the cytosol with chaperones. Hormone binding lets the receptor bind a specific DNA sequence, a hormone-response element, and recruit coactivators or corepressors. The response is a change in transcription and is slower than a GPCR ion-channel response, taking minutes to hours, but it is also stable and genome-wide. Not every lipid-soluble signal works this way. Some have plasma-membrane receptors as well, which is why the speed of a response is evidence about the mechanism.

Nitric oxide is a different small-molecule signal worth one paragraph. It is a gas, made by nitric-oxide synthase in response to Ca2+, and it diffuses into neighbouring smooth-muscle cells, activates guanylyl cyclase, raises cGMP, and relaxes the muscle. This is the basis of local control of blood flow and of the action of nitrovasodilator drugs. cGMP is destroyed by a phosphodiesterase, the target of drugs used to prolong the signal.

12.4.2 The same machinery serves the other chapters #

Signalling is not a separate subject from the rest of cell biology. Integrins signal attachment through some of the same kinases. The Rho-family GTPases that organise the cytoskeleton are switched by receptors. p53 and the CDKs are downstream of growth-factor pathways and of damage sensors. Apoptosis is triggered by both external death receptors and internal stress pathways. When a later chapter names a response, the receptor logic of this chapter is how the cell decided.

12.5 Key terms, summary, and self-check #

Key terms. Paracrine; endocrine; GPCR; Gαs; cAMP; protein kinase A; PLC; IP3; DAG; protein kinase C; receptor tyrosine kinase; SH2; Ras; MAP kinase; PIP₃; Akt; PTEN; hormone-response element.

Summary. Receptors convert a ligand into an intracellular pulse, amplified by G proteins or by kinase cascades and ended by GTP hydrolysis, phosphatases, and endocytosis. GPCRs use cAMP, IP3 and DAG, or direct channel gating. Receptor tyrosine kinases dock adaptors that activate Ras–MAPK proliferation signalling and PI3K–Akt survival signalling. Hydrophobic hormones bind intracellular receptors and change transcription.

Self-check.

1. Adrenaline acts in seconds on a liver cell’s glycogen metabolism, and a steroid hormone acts over hours on the same organ. Which receptor class fits each time scale, and why?

2. Cholera toxin keeps Gαs in the GTP form. What happens to cAMP in the intestinal epithelium, and why is the result water loss?

3. Why is a Ras mutation that blocks GTP hydrolysis oncogenic even when no growth factor is present?

4. What does PTEN remove, and why is its loss similar in outcome to an overactive PI3K?

5. A transcription factor sits in the cytosol until a signal arrives, then enters the nucleus. Which structure from Chapter 9 must the signal expose or create?

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