Cell signalling L1: Unicellular organisms

Communication

Transfer of information

What communication allows us to do

• Individual cells respond to stimuli from:

  → Other cells

  → Environment

What communication used for in multicellular organisms

• control development

• control activity of specifialsed cells

Signalling pathways done with…

Signal transduction

Structure of communication: 1 The pathway

1. Source transmitter generates signal

2. signal transmitted (with communication channel)

3. Receiver intercepts signal

Structure of communication: 2 start→ finish

• information is transmitted when an event in the source reliably produces a response in the receiver

  → input results in output signal

Structure of communication: 3 Temporary signal

• Signals are desired to be transitory:

  → since no information transmitted when the source and receiver are constantly in the same state

Structure of communication: 4 Protein properties

reliable communication results from 2 protein properties

1. binding sites with high affinity and great specificity

2. Conformational changes can alter these binding site activity

How does transmitter transmit signal

1. release diffusible signal molecule

2. through cell surface interactions (e.g surface bound signalling molecule)

How do recievers recieve the signal?

1. Receptors

2. target cell play key role in signal transduction

Target cells only respond to signal if

1. Signal is present

2. Receptor present

Signalling pathways for different distances use different chemical signals

1. Long distance→ pheromones and hormones

2. Intermediate distances→ local hormones

3. Very short distance→neurotransmitters, cell surface molecules and intercellular messengers

Distance ranges: nm→km

Features of the different chemical signals for their distances

Differ in:

• molecular size

• charge

• hydrophobicity

• volitlity

→ influences their biological activity

Examples of these chemical signals

1. Pheromones and chemoattractants

   • cyclic AMP

   • Bombykol

2. Hormones

   • insulin, testosterone, ethylele, ABA, gib

3. Local hormones

   • nitric oxide

   • prostagladin

4. Neurotransmitters

   • ACh, Adrenaline

5. Surface molecules

   • delta ligand

Pheromones

• transmit info between organisms

  → e.g important for sexual reproduction

Examples of Pheromones (3)

1. Insects (silk worm)

   • sexual attraction

   • scent gland on abdomen release bombykol

   • volatile→ move through air up to 1km

   • attract male moths

2. Mammals (odorants)

   • food detection, sexual attraction, recognition, parental behaviour

   • (debated in humans)

   • olfactory endothelium in nose, thousands on sensory neurone→ odour receptors

   • Combinations→ millions different chemicals to smell

3. Slime mould

   • When conditions unfavourable

   • Free living amoebae→ aggregate fruiting body

   • Use cyclic AMP for chemotaxis to aggregate

Slime mould continued→ 2 forms

Suitable environment

• spores germinate to form free- living amoebae

• vegetative form where free-living amoebae move over the forest floor feeding on bacteria

Unsuitable environment (no food)

• stop dividing

• aggregate together to for fruiting body

• develops to contain dormant spores

  → these then used when environments suitable again

Slime mould continued→ Chemotaxis

1. Founder cells secrete cyclic AMP

2. chemoattractant→ directs neighbouring cells to aggregation centre

3. Detection of cAMP by membrane bound receptors

4. formation of pseudopod

5. movement towards the cyclic AMP source

Can be induced experimenting with pulse of cAMP with micropipette

Hormones

• released from one cell type to influence activity of distal target cells

Hormones in plants how spread

• bulk movement of

  • water

  • photosynthate

Hormones in animals how spread

• circulatory systems

Types of hormones

1. Protein hormones (e.g growth factor)

   • synthesised and secreted by ER-golgi-secretory pathway

2. Steroid (hydrophobic) hormones

   • made by enzymes on smooth ER

   • diffuse into circulatory system

Examples of hormone functions and sources in mammal

Local hormones where act?

• Immediate vicinity of secreting cell

• transmit info to

  → neighbouring cells paracrine

  → back to themselves autocrine

How local hormones spread

• diffusion

How does this diffusion affect their action?

• Effect is limited

• due to rapid inactivation

VIP (vasoactive intestinal polypeptide) is considered a local hormone

Local hormones examples

• NO (nitric oxide) → smooth muscle contraction

• Prostaglandins→ blood clotting

• VIP (vasoactive intestinal polypeptide) is considered a local hormone

Neurotransmmitters where released

• released from nerve ending

• to activate target cells:

  → other nerves

  → muscle cells

  → other neurons

Structure of a neurone

1. Dendrites→receive signals from other neurones

2. Cell body→ contains the nucleus

3. Axon→ long thin fibre that conduct AP at 10-100m/s

4. Axon Terminals→ where neurotransmitters are stored and released

Where are neurotransmitters made and released?

• small molecules e.g ACh

• synthesied and packaged at nerve terminal

E.g signal transduction process at neuromuscular junction

1. ACh made

2. Packaged into synaptic vescile

3. AP arrives→ invokes Ca2+ ending

4. Stimulate exocytosis

5. ACh diffuses across synaptic cleft

6. Binds to nicotinic receptors

7. Depolarisation of membrane→ muscle contraction

Cell surface molecules where found

• great variety in multicellular organisms

Cell surface molecules function

• cell-cell interactions

or

• cell-matrix interactions

Important roles of these interactions (2)

1. Cell adhesion→ hold cells together and attached o extracellular matrix

2. (using adheive properies) Signalling functions

   • allows cells to communicate cell-cell or receive information from the cell matrix

Example of cell-cell signalling: Notch-Delta Development of neural cells

Development of neural cells from epithelial in drosophila

1. developing neurall cells presnts a transmemrbane protein: delta

2. Acts as a ligand for transmembrane receptors Notch

   • on neighbouring epithelial cells

3. Activation of the Notch receptors prevents the cell from becoming a neurone

Intercellular messengers

• messengers from one cell to another directly

Types of intercellular messengers

1. Gap junctions

2. Plasmodesmata

Gap junctions

• formed where membranes of opposing cells are joined by cylindrical channels of 1.5-2nm in diameter: Connexons

• Each spans one membrane so two form on top of eachtoher to form a pipe line between the two cells

• Each connection is composed of 6 subunits

  → Forms as channel of 1.5 nm in diameter

What can pass the gap junctions

• ions

• small molecules of MW < 2000

• Other messengers:

  → cAMP, IP3 and Ca2+

  • diffuse from one cell to the next

Example of gap junction use

• used in depolarisation spread in cardiac muscle cells

The permeability of gap junctions is regulated

Decrease in cytostolic pH/ increase in Ca2+:

• reversibly decreases permeability

WHY?

• Suggests there is cell death next door

Plasmodesmata

Specialised junctions found in plants:

• Tubular extension of plasma membrane plasmodesma

• 40-50nm in diameter

• Extensions of endoplasmic reticulum desmotubules pass through plasmodesma

What do desmotubules do

• allow diffusion of molecules up to 1000 Daltons

  → Allows for intercelular transport of metabolites, signalling molecules and proteins

Self assessment questions:

1. What is cell signalling?

2. How is information transduced in signalling systems?

3. How do cells transmit signals?

4. What do cells use to receive signals?

5. Over what scales does cell-to-cell signalling occur?