Lec 2 - Cell signalling II

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Endocrine functions

Endocrinology = Communication systems & information transfer in body

Functions of endocrine signals include:

Maintain internal homeostasis

Support cell growth

Co-ordinate development

Co-ordinate reproduction

Facilitate responses to external stimuli

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Signalling

i) Endocrine vs Synaptic signalling

Endocrine signals tend to be long-range signals - released 'far' from target site

Hormones

Yet conc. in blood tends to be low...

Why?

Their receptors have high-affinity for them!

Synaptic signals are short-range signals - released close to their target site (within synaptic cleft)

Neurotransmitters

Conc. in synaptic cleft tends to be high...

Why?

Their receptors have relatively low-affinity for them

Allows ligand to dissociate rapidly from receptor to help terminate response

ii) Speed of response

NTs have shorter diffusion distance (<100nm) to target

Response takes miliseconds

Hormones have longer diffusion distance (varies from cms to metres) to target

Response takes seconds, minutes, hours

What determines speed of response?

Gene expression!

If response requires changes in proteins already present = rapid

If response requires alteration in gene expression + synth. new proteins = slower

iii) Multiple signals

Cells often respond to multiple signals

How do different types of cells respond to the same extracellular signal molecule?

Take Acetylcholine

It can bind to a variety of receptors on many different types of cell

Yet all the cells do not respond in the same way!

iv) Morphogen gradient

Concentration gradient that determines the location, differentiation & fate of developing cells

Morphogen = secreted signalling molecule

Concentration differences along gradient tend to be v small

Cells are able to respond appropriately depending on conc. perceived by them:

Receptors on surface transduce info to nucleus

Create appropriate gene/cell response

V important in animal development

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Receptors

i) What are receptors?

Protein/protein complexes which contain a binding site

Allows messenger-ligand attachement

Transmission of message to internal cell structures

2ndary binding sites may interact with either another protein or DNA

ii) Types of receptors

2 main classes:

Plasma membrane receptors

Intracellular (nuclear) receptors

iii) Binding sites

Chemical messenger secreted in response to stimulus

Messenger transported via ECF to target cell

Receptor-ligand binding

Governed by specificity

Receptor-ligand complex causes response

Receptor will initiate a particular signal transduction pathway

Signal termination

Failure to terminate signal = cause of many major diseases, including cancer

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Signal termination

In cellular biology - for every activation there must be a deactivation!

Both are important in process of cell signalling

Usually metabolic:

Proteolysis

Hydroxylation

Conjugation

Excretion of metabolites usually follows signal termination

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Chemical messengers

i) Hormones derived from amino acids

ii) Hormones derived from lipid precursors

iii) Hormones derived from other chemicals