Week 7 cell communication and signalling

Question 1

Intro to cell signalling 1

Answer 1

Cell-to-cell communication is essential for every organisms
It coordinates cell activities within tissues/organs in multicellular organisms, in response to external/internal changes

Question 2

Intro to cell signalling 2

Answer 2

Cells communicate through physical interactions or signalling molecules (Cell
signalling)

Question 3

Stable cell-cell interactions

Answer 3

For cell adhesion within a tissue
-Tissue organisation (e.g. Epithelia) and cell polarity
-Anchoring cells to the ExtraCellular Matrix (space between cells)

Question 4

Three Types of Junctions (multi protein complexes)

Answer 4

-Gap junctions
-Anchoring junctions (desmosome)
-Tight junctions

Question 5

Gap junctions

Answer 5

Communicating junctions
->Cylindrical channels on the plasma membrane of 2 adjacent cells forming a pore - connexons of 6 connexin proteins each
->Direct and bidirectional molecule exchange -> heart (cardiac muscle): to pass the signal to contract
->No effect in cell-extracellular matrix adhesions
->ONLY GAP JUNCTIONS provide direct communication or material exchange between cells

Question 7

Cell signalling

Answer 7

The signalling cell releases/
exposes outside a specific signal molecule that is detected by the target cell
Target cells must possess specific receptors to recognise the signal
Signals can act over a long or
short range
5 methods of cell signalling;
-Contact-dependent
-Paracrine
-Synaptic
-Endocrine
-Autocrine

Question 8

Contact-dependant signalling

Answer 8

Through direct cell-to-cell contact, as;
Between membrane-bound signal molecules on a
signalling cell and receptors on an adjacent target cell,
-Signals are not secreted
-Signals exchange via gap junctions

Question 9

Endocrine signalling

Answer 9

Signals (hormones) to act on distant cells;
-Release of hormones by endocrine glands into the
bloodstream to act on target cells at distant body sites

Question 10

Paracrine signalling

Answer 10

Signals that act over a short distance;
Release of signalling molecules by a cell to act on neighbouring target cells within a local area

Question 11

Synaptic signalling

Answer 11

Specific to neurons
It occurs across synapses, small gaps between neurons or
between neurons and target cells
The signalling molecules are neurotransmitters, which are
released from the axon terminal of a neuron
Neurotransmitters diffuse across the synapse and bind to
receptors on the postsynaptic cell

Question 12

Autocrine signalling

Answer 12

Signals that act back on the secreting cell
-When a cell produces and releases a signal that acts on the same cell (signalling cell=target cell)
-Feedback mechanism to self-regulate cellular processes

Question 13

Cell signalling pathway

Answer 13

3 stages;
-Signal-receptor binding
-Signal transduction - to convert and amplify the extracellular signal into a cascade of intracellular events
-Specific cell response - a signal molecule can induce different responses in different cells

Question 14

Signal - receptor binding

Answer 14

Receptors are proteins that recognise specific ligands (complementary shape) and transmit the signal intracellularly (for signal transduction)
High specificity -> one (or a few) of signal(s) only

Question 15

Signal receptor interaction - intracellular receptors

Answer 15

-Small hydrophobic signals

Question 16

Signal - receptor interaction - cell surface receptors

Answer 16

-Hydrophilic signals

Question 17

Intracellular receptors

Answer 17

-Found in the cytosol or nucleus of target cells
-Small / hydrophobic hormones can diffuse across the plasma membrane and bind to the receptor
Hormone-receptor complex form a transcription factor
->Binding DNA sequences, to modify the transcription level of specific genes
->To alter effector proteins synthesis as a response

Question 18

Types of membrane receptors

Answer 18

Three main types of membrane receptors;
-Ion channel-coupled receptors
-G protein-coupled receptors (GPCRs)
-Enzyme-coupled receptors
Signal-receptor interaction ALWAYS induces a conformational change of
the receptor, that leads to the transmission of an intracellular signal

Question 19

Ion channel-coupled receptors

Answer 19

-Signal-receptor binding opens or closes an ion channel -> allowing specific ions (e.g., Na⁺, K⁺, Ca²⁺, Cl⁻) to pass through
-Ion flow changes the intracellular charge, altering the membrane potential
-Converting chemical signals into electrical ones (nerve impulse conduction)

Question 20

G proteins -> coupled receptors

Answer 20

G proteins-linked receptors have a common structure, with 2 components;
->Trans-membrane receptor
->G protein

Question 21

Trans-membrane receptor

Answer 21

-Trans-membrane protein that crosses 7 times the plasma membrane
-Has an extracellular ligand-binding portion

Question 22

G protein

Answer 22

->Acts as an on/off switch
-Integral membrane protein
located on the cytoplasmic side
-Composed of 3 protein subunits (α, β, γ)
-When inactive, the α subunit binds GDP
-Signal-receptor binding causes α subunit to exchange GDP for GTP (active state)

Question 23

G proteins-coupled receptor activation

Answer 23

Signal binding -> receptor conformational change
->G-protein is attracted to GPCR and activates its α subunit to exchange GDP for GTP
->α subunit dissociates from the βγ complex

Question 24

G proteins-coupled receptor effect and inactivation

Answer 24

Both α subunit and a βγ complex can interact and activate ion channels or membrane-bound
enzymes to activate signalling pathways
->GPCRs INACTIVATION

Question 25

Enzyme-coupled receptors

Answer 25

Similar to GPCRs, but the receptor cytoplasmic part either:
acts as an enzyme itself ->receptor tyrosine kinases, RTKs
Or forms a complex with an enzyme

Question 26

Receptor tyrosine kinases (RTKs)

Answer 26

Membrane receptors that attach phosphates to tyrosine amino acids
-Can trigger multiple signal transduction pathways at once
-Usually, stimulating cell growth and cell survival
Abnormal functioning of RTKs is associated with many types of cancers

Question 27

Enzyme-coupled receptors

Answer 27

1) Signal binding induces the pairing of 2 receptors (dimerisation)
2) Intracellular receptor kinase portion phosphorylate each other’s specific tyrosines
3) Phosphorylated tyrosine recruit and activate (phosphorylation) signalling proteins
4) Activated proteins trigger cell complex responses
The effect is reversed by phosphatases removing the phosphates from tyrosine

Question 28

2- Transduction step

Answer 28

-Multistep process to amplify the signal intracellularly -> opportunities for regulation
-Different transduction strategies
-Phosphorylation cascade
-Release of second messengers

Question 29

2a- Protein phosphorylation cascade

Answer 29

->Phosphorylation cascade
In many pathways, the signal is transduced by a protein phosphorylation cascade
Molecular switch between phosphorylation and dephosphorylation of
signalling proteins -> turning their activities on and off

Question 30

2b- Second Messengers

Answer 30

Release of second messengers
-Extracellular signal molecule - receptor binding -> pathway’s “first messenger”
-Signal transduction of some pathways can activate the release of second messengers, which can readily spread through cells to amplify the signal -> in most cases initiated by G-protein-linked receptors and receptor tyrosine kinases
-Second messengers are small, nonprotein, water-soluble molecules or ions
e.g., cAMP (cyclic AMP), cGMP (cyclic GMP), lipids, Ca2+, NO (nitrogen monoxide)

Question 32

Step 3 - cellular response

Answer 32

-Cell signalling ultimately leads to the regulation of one or more cellular activities
Regulation of gene expression (increase or decrease transcription of target genes) is a common outcome