Cell Communication

Types of cell communication

Direct contact between cells

Through extracellular chemical messengers

Local Regulators

What are the factors of direct contact between cells

Gap junctions

Cell-to-cell recognition

Gap junctions

* cells are connected by tunnels formed by connexons
* Allows ions and small water soluble chemicals to pass between cells
* Cardiac and smooth muscle

Cell-to-cell recognition

* interaction of cell-surface molecules


* Immune cells – recognize self vs. non-self


* Receptors meet glycocalyx

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How does cell communication occur through extracellular chemical messengers?

* Cells release ligands that bind to receptors on target cells to initiate a desired cellular response
* Local regulators vs. Long distance regulators

How does cell communication occur through local Regulators

Paracrine

Autocrine

Ex: histamine, cytokines, prostaglandins

Paracrine

through diffusion, ligands affect cells in the **local** vicinity

Autocrine

chemicals act on the cell that produced it

Neurotransmitters

* secreted by neurons
* Diffuse across the synaptic cleft and target the adjacent cell (neuron, gland, muscle)
* Short-lived

Long-range (distant) regulators

Hormones

Neurohormones

Hormones

secreted into the blood by endocrine glands to travel to distant target cells

Neurohormones

neurons secretes hormones into the blood

How do hormones effect cells?

* Activate or inhibit enzymes
* Direct protein synthesis through activation of transcription factors
* Stimulate cell division
* Alter membrane permeability – membrane potential or opening/closing of ion channels

How do hormones work?

* chemical substances that travel through the blood to a target cell
* Target cells must have specific receptors to which the hormone binds
* These receptors may be intracellular or located on the plasma membrane


* They trigger a change in cellular activity

Types of Hormones

Amines – amino acid derivatives

Peptides – proteins

Steroids – cholesterol based

Eicosanoids –derived from arachidonic acid (lipid)

Transport of Hormones

Hydrophilic (water-soluble) hormones – dissolve in the plasma

Lipophilic (lipid-soluble) hormones – circulate bound to plasma proteins such as albumin

Mechanism of Signaling Molecules

Lipid soluble hormones

Water soluble hormones

Lipid soluble hormones

* Move through the plasma membrane and bind to an intracellular receptor in the nucleus


* Binding of the HRC to the DNA triggers transcription of a specific gene and the synthesis of a protein that initiates a cellular response

Water soluble hormones

* **utilize a membrane bound receptor**
* Binding of hormone causes a conformational change in the receptor

Two affects of ligands binding to membrane bound receptors:

Triggers the opening or closing of ion channels or transfers the signal to a secondary messenger within the cell which triggers a cascade of biochemical events

Types of Receptors 

Ligand gated channel

Receptor enzyme

G protein coupled receptor

Integrin

Ligand gated channel

Open or close the channel

Receptor enzyme

Ligand binding to a receptor enzyme activates an intracellular enzyme

G protein coupled receptor

Ligand binding to a G protein-coupled receptor opens an ion channel or alters enzyme activity

Integrin

Ligand binding to integrin receptors alters the cytoskeleton

Signal Transduction

* Amplifier enzyme activates several more molecules.
* Protein Kinase transfers phosphate from ATP to protein.
* Ca2+ binds to protein and alters the protein function.

Secondary Messenger Pathways

**G-protein linked receptor**

**G proteins**

G-protein linked receptor

* spans the membrane and is associated with a G protein on the cytoplasmic side
* Ligand binds to the receptor
* Receptor then activates a G protein

G proteins

relay proteins that cycle between an inactive and active form

Cyclic AMP Secondary Messenger Pathway steps 1-4

1. G protein activates the enzyme adenylate cyclase (amplifier)
2. Converts ATP to cyclic AMP
3. Activates protein kinase A which phosphorylates other proteins
4. Trigger cellular responses

Calcium Secondary Messenger Pathway steps 1-2

1. Binding of ligand to a G protein linked receptor or a tyrosine kinase receptor

2\.  Activates phospholipase C (amplifier) which cuts PIP2(phospholipid) to DAG and IP3

Calcium Secondary Messenger Pathway steps 3-5

3. DAG will activate protein kinase C in another signaling pathway
4.   IP3 binds to calcium channels triggering the release of calcium from the SER into the cytoplasm
5. Calcium binds to and activates calmodulin (protein) which can alter other proteins to bring about a cellular response