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K.Muma
Bio 6
Cell Communication
Study Objectives:
1.
Describe the different mechanisms of cell communication: direct contact vs.
extracellular messengers.
2.
Compare and contrast paracrine, autocrine, neurotransmitters, hormonal, and
neurohormonal communication.
3.
Discuss the general mechanism of hormone action and list the effects they can
have on cellular activity
4.
Distinguish between lipid soluble and water soluble hormones. Describe how
they are transported in the blood and the type of receptors they bind to at the
target cells.
5.
Define and give examples of secondary messengers.
6.
Describe the general mechanism of the cyclic AMP signal transduction
pathway.
7.
Describe the general mechanism of the calcium signal transduction pathway.
Cell Communication Outline
I.
Types of Cellular Communication (See figure 4-20)
a. Direct contact between cells
i. Gap junctions – cells are connected by tunnels formed by
connexons
1. Allows ions and small water soluble chemicals to pass
between cells
2. Examples: cardiac and smooth muscle
ii. Cell to cell recognition – interaction of cell-surface molecules
1. Example: immune cells – recognize self vs. non-self
b. Through extracellular chemical messengers - cells release ligands that
bind to receptors on target cells to initiate a desired cellular response
i. Local regulators
1. Paracrine – through diffusion, ligands affect cells in the
local vicinity
a. Examples: histamine, cytokines, prostaglandins
2. Autocrine – chemicals act on the cell that produced it
3. Neurotransmitters – secreted by neurons
a. Diffuse across the synaptic cleft and target the
adjacent cell (neuron, gland, muscle)
b. Short-lived
ii. Long-range (distant) regulators
1. Hormones – secreted into the blood by endocrine glands to
travel to distant target cells
2. Neurohormones – neurons secretes hormones into the
blood
II.
Cellular Effects of Chemical Signals
a. Example of various effects chemical signals may have on the target cell
i. Activate or inhibit enzymes
ii. Direct protein synthesis through activation of transcription factors
iii. Stimulate cell division
iv. Alter membrane permeability – membrane potential or
opening/closing of ion channels
b. How do hormones work?
i. Hormones are chemical substances that travel through the blood to
a target cell
ii. Target cells must have specific receptors to which the hormone
binds
iii. These receptors may be intracellular or located on the plasma
membrane
iv. They trigger a change in cellular activity
c. Types of Hormones/Chemical Signals
i. Amines – amino acid derivatives
1. Examples: catecholamines (NE and E), T3, T4, serotonin,
melatonin
ii. Peptides – proteins
1. Examples: vasopressin (ADH), insulin
iii. Steroids – cholesterol based
1. Examples: cortisol, aldosterone, estrogen, testosterone
iv. Eicosanoids –derived from arachidonic acid
1. Prostaglandins, leukotrienes
d. Transport of Hormones
i. Hydrophilic (water-soluble) hormones – transported dissolved in
the plasma
1. Examples: peptide and catecholamines
ii. Lipophilic (lipid-soluble) hormones – circulate bound to plasma
proteins such as albumin
1. Examples: steroids and thyroid hormones
e. Mechanism of Signaling Molecules
i. Lipid soluble hormones (see figure 4-28)
1. Move through the plasma membrane and bind to an
intracellular receptor in the nucleus
2. Binding of the hormone receptor complex to the DNA
triggers transcription of a specific gene and the synthesis of
a protein that initiates a cellular response
ii. Water soluble hormones- utilize a membrane bound receptor
1. Binding of hormone causes a conformational change in the
receptor
2. Affects of ligands binding to membrane bound receptors
(see figure 4-22):
a. Triggers the opening or closing of ion channels
OR
b. Transfers the signal to a secondary messenger
within the cell which triggers a cascade of
biochemical events
III.
Signal Transduction
a. Secondary Messenger Pathways
i. Signaling pathways amplify the cells response to signals
ii. G-protein linked receptor – spans the membrane and is
associated with a G protein on the cytoplasmic side (see figure 422c)
1. Ligand binds to the receptor
2. Receptor then activates a G protein
iii. G proteins are considered relay proteins that cycle between an
inactive and active form
b. Types of secondary messenger pathways
i. Cyclic AMP Secondary Messenger Pathway (see figure 4-25)
1. G protein activates the enzyme adenylyl cyclase (the
effector)
2. Converts ATP to cyclic AMP
3. Activates protein kinase A which phosphorylates other
proteins
4. Trigger cellular responses
ii. Calcium Secondary Messenger Pathway (see figure 4-26)
1. Binding of extracellular messenger to a G protein linked
receptor or a tyrosine kinase receptor
2. Activates phospholipase C which converts PIP2 to DAG
and IP3
3. DAG will activate protein kinase C in another signaling
pathway
4. IP3 binds to calcium channels triggering the release of
calcium from the ER into the cytoplasm
5. Calcium binds to and activates calmodulin which can alter
other proteins to bring about a cellular response
Post-lecture Practice
1. When a ligand triggers a secondary messenger pathway a cascade of events
occurs. Place the following events of the calcium signaling pathway in the proper
sequence 1 being the first event and 9 being the last.
_____ IP3 binds to an IP3 receptor on the smooth endoplasmic reticulum
_____ G-protein is activated
_____ the effector phospolipase C is activated
_____ ligand binds to G-linked protein receptor
_____ calcium is released from the smooth ER and dumped into the cytosol
_____ transcription factors are activated
_____ cellular response occurs
_____calcium activates calmodulin
_____ PIP2 is converted into DAG and IP3
2. Categorize the following signaling molecules as local regulators or distant
regulators:
Chemical signaling through gap junctions
Paracrine
Neurotransmitters
Hormones
Neurohormones
Cell to cell recognition
Autocrine