Cell to Cell Communication

Chapter 11

Example of Cell to Cell Communication

• Yeast’s version of sex• Two types of cells (a and )• Each secretes a mating factor that

binds to receptor on opposite cell• Binding of mating factors lead to cell

growth and fusion• Nucleus of fused cell contains DNA

from both a and .

Figure 11.0 Yeast

Figure 11.1 Communication between mating yeast cells

Local and Long Distance Signaling

• Local signaling – influences cells in the nearby vicinity– Paracrine signaling – secreting cell releases

a regulator in the the extracellular matrix– Synaptic signaling – nerve cell releases

neurotransmitter into a synapse (space between nerves)

• Long Distance signaling – can influence cells all over body– Hormone signaling – endocrine cells

secrete hormones into blood where they can reach any cell

Figure 11.3 Local and long-distance cell communication in animals

Figure 11.4 Communication by direct contact between cells

Cell junctions and cell-cell recognition

Three Stages of Cell Signaling

• Reception – target cell’s detection of a chemical signal– Signal is detected when it binds to a

receptor– Ligand – a signal molecule that binds to a

receptor• Transduction – binding of signal to receptor

stimulates a change in the receptor. – The changed receptor triggers a step or

many steps that lead to the cell response.• Response – end result – the cell response

Figure 11.5 Overview of cell signaling (Layer 1)

Figure 11.5 Overview of cell signaling (Layer 2)

Figure 11.5 Overview of cell signaling (Layer 3)

Receptors

• Intracellular receptors – found in cytoplasm or on nucleus so signal must pass through cell membrane first

– Ex. NO and steroid hormones like testosterone

– Testosterone receptor only found in certain cells

• An activated testosterone receptor acts as a transcription factor

– Transcription factors - turn on or turn off genes

Figure 11.10 Steroid hormone interacting with an intracellular receptor

• Cell Membrane Receptors – found in cell membrane

– Three major types

• G-linked receptor

• Receptor tyrosine kinase

• Ligand-gated ion channel

Figure 11.6 The structure of a G-protein-linked receptor

Figure 11.7 The functioning of a G-protein-linked receptor

Figure 11.8 The structure and function of a tyrosine-kinase receptor

Figure 11.9 A ligand-gated ion-channel receptor

Transduction

• Signal transduction pathways – a chain of molecular interactions (like falling dominoes)

• Often involves protein phosphorylation and dephosphorylation– Protein kinases – enzymes that transfer

phosphate groups from ATP to a protein– Phosphorylation of protein often changes

protein from inactive to active form– Protein phosphatases – enzymes that

rapidly remove P (often turn off pathway)

Figure 11.11 A phosphorylation cascade

Second Messengers

• Second messengers – small, nonprotein, water-soluble molecules or ions that are part of signaling pathways

• Two most common second messengers

– Ca2+

– Cyclic adenosine monophosphate (cAMP or cyclic AMP)

cAMP

• Adenylyl cyclase – an enzyme in cell membranes that converts ATP into cAMP

• Phosphodiesterase – an enzyme that converts cAMP into AMP

Figure 11.12 Cyclic AMP

Figure 11-12x cAMP

Figure 11.13 cAMP as a second messenger

Ca2+

• Second messenger involved in growth factors, some hormones, muscle contractions, neurotransmitters, and cell division

• Ca2+ levels are high in the blood, ER, and sometimes mitochondria and chloroplasts

• Low Ca2+ cytosol concentration allows even small fluctuations to trigger pathways

Figure 11.14 The maintenance of calcium ion concentrations in an animal cell

Figure 11.15 Calcium and inositol triphosphate in signaling pathways (Layer 1)

Figure 11.15 Calcium and inositol triphosphate in signaling pathways (Layer 2)

Figure 11.15 Calcium and inositol triphosphate in signaling pathways (Layer 3)

Figure 11.16 Cytoplasmic response to a signal: the stimulation of glycogen breakdown by epinephrine

Figure 11.17 Nuclear response to a signal: the activation of a specific gene by a growth factor

Fine Tuning of Response

• Signal Amplification – number of activated products is much greater than in preceding step

• Specificity – different cells have different proteins so two cells can respond to same signal in different manner– Ex. epinephrine stimulates liver to break

down glycogen and heart cells to contract faster

• Scaffolding Proteins – large relay proteins to which several other relay proteins are attached

Figure 11.18 The specificity of cell signaling

Figure 11.19 A scaffolding protein

Apoptosis• Apoptosis - triggered by signals that

activate cell suicide

• In C. elegans (small worm), death genes called ced. When activated they stimulate death by activating proteases and nucleases.

• Very similar genes found in other animals including humans

• Apoptosis problems are associated with cancer, Parkinson’s, and alzheimer’s