Cell signalling

Question 1

Describe the structure of membrane receptors

Answer 1

Cell-surface receptors are integral membrane proteins that span the plasma membrane.

Question 2

What is the importance of signalling in controlling cell behaviour?

Answer 2

Cell signalling controls growth, cell differentiation and development, metabolism and apoptosis. It also allows cells to interact with their environment and neighbouring cells. Defective signalling often results in disease.

Question 3

List the different types of signal that a cell may encounter

Answer 3

Autocrine: feed-forward mechanism where cell releases signal which binds on the surface of that cell.

Paracrine: local signalling where a transmitter released from one cell to another neighbourign cell

Endocrine: transmitter or signal released at distal sites which travel to target cells

Cell-cell: membrane bound signals on one cell bind receptors on the surface of an adjacent target cell.

Question 4

What are the five different types of cell signalling receptor?

Answer 4

Intracellular receptors (transcription factors, enzymes)

Receptors that are ion channels

Receptors with intrinsic enzyme activity

Receptors linked to soluble protein kinases

Receptors coupled to targets via G-proteins

Question 5

List the three common mechanisms cells use to change behaviour

Answer 5

Alter gene transcription

Alter enzyme activity

Alter ion balance across the membane

Question 6

What is the advantage of a multiple component signalling pathway over a single component pathway?

Answer 6

Multiple component signalling pathways allow amplification and integration of signalling cascades, whereas single component systems are either off or on depending on the presence of the signal.

Question 7

Describe the role of second messengers in signalling

Answer 7

Second messengers facilitate amplificaion of an extracellular signal.

Second messengers are small, metabolically unique molecules. The binding of a ligand to a receptor leads to a rapid change in concentration from high to low. These then activate target proteins which alter the behaviour of the cell.

Question 8

Discuss the importance of Ca²⁺ as a second messenger

Answer 8

The concentration of free Ca²⁺ in the cytosol is kept very low (maintained by ATP pumps that transport Ca²⁺ out or into the ER). Following a signal induced release there can be a 100-fold increase in Ca²⁺ concentration. Ca²⁺ can bind to and activate a variety of target proteins.

Muscle: triggers contraction
Nerves and Endocrine cells: induces vesicle fusion and exocytosis
Binds calmodulin and other Ca²⁺-binding proteins causing a conformational change which allows binding to other target proteins.

Question 9

Discuss transcription factors as intracellular recptors for steroid hormones`

Answer 9

Transcription factors are ligand-activated receptors that transduce signals by modifying gene transcription. Steroid hormone receptors are found in the cytoplasm in the absence of their ligand.

Sterioid hormones (e.g. oestrogen, cortisol, progesterone, aldosterone) enter the cell by diffusion through the plasma membrane and bind to their receptor. The transcription factor then becomes activated and translocates to the nucleus where it activates or suppresses gene transcription by binding to response elements in the DNA.

Question 10

Name the types of ligand that can bind to intracellular receptors (4)

Answer 10

Transcription factors: steroid hormines, small lipophillic molecules

Enzymes: NO binding to guanylate cyclase

Question 11

Describe the nitric oxide in vasodilation

Answer 11

Nitric oxide produced by endothelilal cells diffuses across the plasma membrane of smooth muscle cells and binds to guanylate cyclase. This converts GTP to cGMP which activates PKG. PKG phosphorylates MLCK, decreasing it’s activity and also promotes the uptake of Ca²⁺ into intracellular stores.

This causes muscle relaxation, dilating the blood vessel

Question 12

Name 5 different types of Ga receptor and their respective actions

Answer 12

1. Gas - adenylate cyclase (glucagon, ACTH)
2. Gai - inhibits andenylate cyclase (PGE1, adenosine
3. Gat: stimulates cGMP phosphodiesterase (rhodopsin)
4. Gaq - stimulates PLC (vasopressein)
5. Ga13 - activates ion channels (Na⁺/H⁺ exchange)

Question 13

What are the five muscarinic Ach receptor subtypes?

Answer 13

M1, M3, M5 couple to Gq, stimulate PLC

M2 couples to Gi, opens K⁺ channel

M4 couples to Gi to inhibit AC

Question 14

Describe the signalling pathway used by atrial natriuretic peptide

Answer 14

ANP is released by atrial muscle in response to stretch (high blood pressure). It binds to the GC-A receptor in kidney cells and other tissues. Guanylate cyclase converts GTP to cGMP, which activates a variety of target proteins including PKG.

PKG phosphorylated Na+ channels and reduces NA reabsorption, induces vasodilation of blood vessels and increases salt exretion and urine production. This reduces blood pressure

Question 15

Describe the signalling pathway used by epidermal growth factor

Answer 15

The epidermal growth factor receptor is a type 1 RTK.

Binding of EDF induces formation of homodimers which activates the receptor tyrosine kinase in each monomer. The receptors undergo mutual phosphorylation of tyrosine residues.

pTyr provide binding sites for adaptor proteins via an SH2 domain. The adaptor proteins contain two SH3 domains which bind proline rich regions in a GEF. GEF activates Ras by exchanging GDP for GTP.

Active Ras initiates a map kinase cascade.

Question 16

Explain how acetylcholine induces the contraction of skeletal muscle but inhibits the contraction of cardiac muscle

Answer 16

Acetylcholine released by the vagus nerve binds to the M2 Ach receptor on cardiac myocytes which is a GPCR with a Gi alpha subunit. Activation of the GPCR opens K⁺ channels in the plasma membrane, and the efflux of K⁺ causes hyperpolarisation so depolarisation is more difficult. Therefore Ach reduces the strength and frequency of contraction.

Ach released by cholinergic neuorons at the NMJ binds to Na/K channels. Na⁺ rapidly enters the cell and K⁺ slowly leaves. Depolarisation opens VG-Na⁺ channels and activates the voltage sensing protein, which opens Ca²⁺ channels. Increased Ca²⁺ induces muscle contaction.

Question 17

Discuss the role of PLC in the activation of PKC

Answer 17

The cleavage of PIP2 by PLC generates two second messengers - DAG and IP3. IP3 is a soluble molecule that diffuses through the cytoplasm and binds to Ca²⁺ channels in the sarcoplasmic reticulum. Activation of Ca²⁺ channels increases [Ca²⁺] in the cytosol. DAG remains bound to the plasma membrane.

PKC has two binding sites on its regulatory subunit, a Ca²⁺ binding site and a DAG binding site. It is held in an inactive state by the regulatory domain which acts as a pseudosubstrate. Increase in intracellular Ca²⁺ stimulated by IP3 changes the conformation of PKC which exposes the DAG binding site and promotes transport to the plasma membrane where it binds DAG and becomes active.

Question 18

Discuss the role of adenylate cyclase in the activation of PKA

Answer 18

Adenylate cyclase converts ATP to cAMP. The regulatory domain of PKA contains a pseudosubstrate which binds to the catalytic subunit. Binding of cAMP causes a change of conformation and releases teh catalytic subunits.

Question 19

Name two excitatory and two inhibitory transmitters

Answer 19

Excitatory: Ach, Glutamate, Serotonin

Inhibitory: GABA, Glycine

Question 20

Describe the signalling pathway used by TGF-beta

Answer 20

The TGF-beta receptor is a serine/threonine kinase. Ligands of Ser/Thr kinases are inhibitory growth factors and have anti-proliferative effects on most cells.

Binding of the ligand induces receptor dimerisation which phosphorylate a SMAD protein. SMAD-P forms a complex with SMAD4 which migrates to the nucleus and activates transcription factors, inducing expression of target genes.

Question 21

What is the effect of ACh binding to the nicotinic ACh receptor?

Answer 21

The nicotinic ACh receptor is a Na/K-ATPase. Na enters the cell and K is moved out, causing depolarisation of the cell.

Question 22

Describe the events at the neuromuscular junction

Answer 22

Action potentials are propogates to the axon terminal of the motor neurone

The presence of an action potential triggers opening of voltage gated Ca-channels, allowing Ca ions into the cell. Ca triggers fusion of synaptic vesicles containing ACh with the plasma membrane.

ACh diffuses across the synaptic cleft and binds to the nACh receptors on the motor end plate which is a Na/K-ATPase. Na+ enters the cell and K+ moves out, resulting in depolarisation. If threshold is reached, an end plate potential results initating an action potential throughout the muscle fibre.

Question 23

What mechanisms are used to ensure that a low intracellular [Ca²⁺] is maintained?

Answer 23

Ca-ATPase pumps calcium from the cytoplasm into the SR forming an intracellular store

Calcium is chelated by intracellular proteins e.g. calmodulin, calsequestrin

Na⁺/Ca²⁺ antiporter on the plasma membrane pumps Ca²⁺ out of the cell.

Ca²⁺-ATPase on the plasma membrane actively pumps Ca²⁺ out of the cell

Question 24

Name 5 channels that increase the cytoplasmic concentration of Ca²⁺

Answer 24

Voltage-gated Ca²⁺ channel (plasma membrane)

Voltage-gated Ca2+ channel (SR)

IP3-gated Ca2+ channel (SR)

IP4-gated Ca2+ channel (plasma membrane)

Ca2+-gated Ca2+ channel (SR)

Question 25

Describe the coupling of dopamine receptors

Answer 25

D1 and D5 - couple to Gs

D2, D3, D4 - couple through Gi

Question 26

Describe the coupling of serotonin receptors

Answer 26

5HT1 couples to Gi

5HT2 couples to Gq

5HT4-7 couple to Gs

5HT3 is a Na/K-channel

Question 27

Describe the coupling of adrenergic receptors

Answer 27

a1 receptors couple to Gq

a2 receptors couple though Gs

beta receptors couple though Gs

Question 28

Compare the activation of PLC by GPCRs and RTKs

Answer 28

Hormonal stimulation of GPCRs which have a Gaq subunit activates PLC to cleave PIP2 into IP3 and DAG. leading to an increase in cytosolic Ca2+ and activation of PKC

RTKs can initiate this pathway by activating a different isoform of PLC which binds to the SH2 domains of activated RTKs and becomes activated. PLC becomes localised to the membrane and is phosphorylated which increases its activity.

Question 29

How is intracellular [Ca2+] regulated?

Answer 29

Actively pumped out of the cell: Ca2+-ATPase (SR and PM)

Removed from the cell by Na+/Ca2+ antiporter

Ca2+ sequestered by proteins calmodulin, calsequestrin

Entry controlled by VD-Ca2+ channels

IP3 and Ca2+-gated Ca2+ channels on the SR, and IP4-gated channels on the PM allow entry of Ca2+

Question 30

What signalling pathways are activated by growth factors?

Answer 30

Growth factors stimulate cell division by binding to RTKs. This causes the receptor subunits to dimerise and undergo autophosphorylation. Phosphorylated tyrosine binds to an adaptor protein which activates Ras. Ras activates the MAPK cascade which phosphorylates transcription factors that stimulate the production of G1 cyclins, therefore promoting cell division.

Inhibitory growth factors bind to Ser/Thr kinases which induces receptor dimerisation and SMAD becomes phosphorylated. SMAD-p forms a complex with co-SMADs which migrate to the nucleus and activate transcription factors of target genes which inhibit cell division e.g. CKIs