Membranes & Cell Signalling
Department of Bioscience

Lecture Notes: Membrane Receptors & Cellular Signalling

Background reading:

Read chapter 20 Lodish, Berk, Zipursky, Matsudaira, Baltimore and Darnell Molecular Cell Biology (4th ed) WH Freeman and Co. 1999 http://www.whfreeman.com/lodish/

chapters 1&2 Hancock, JT Cell Signalling. Pearson Education Ltd.: Harlow, England. 1999 (library, short loan section).

Aspects of Signalling

Cells need to communicate in order to co-ordinate their activity There are two types of communication
  1. electrical
  2. chemical
The advantages of chemical signals Eukaryotic microorganisms (yeast, slime molds, protozoans) - Secreted molecules coordinate the aggregation of free-living cells for mating (sexual reproduction) or for differentiation under certain environmental conditions. Pheromones are chemicals released by one organism which alter the behaviour or gene expression of others of the same species (eg yeast mating-type factors). Pheromones are used by multicellular organisms also.

Signalling molecules which function within an organism control many processes for normal activity

What makes a good signalling molecule? Each step in the process is requlated
  1. synthesis
  2. release
  3. transport
  4. detection by target cell (via a receptor)
  5. change in cellular metabolism, function or development
  6. deactivation of signal (which often terminates the cellular response)
Several types

Signals between cells

There are many routes by which extracellular signals reach their targets

Some compounds act in more than one type of cell-to-cell signalling
Examples:

Extracellular signalling molecules

  1. lipophilic hormones
  2. water soluble compounds (range from small charged biogenic amines to large peptide hormones)

Lipophilic hormones
Lipophilic hormones (steroids, retinoids) diffuse across membrane of target cell and activate intracellular receptors. Usually stored as precursor rather than mature, active hormone. Effects take a while to occur and persist from hours to days.


Lipophilic hormones, such as the prostaglandins (eicosinoids), derived from arachadonic acid


Biogenic amines and peptides


Biogenic amines (neurotransmitters and hormones)

Regulation of synthesis, release and degradation of hormones and neurotransmitters enables ability to respond rapidly to changes in internal or external environment. The actions of these signalling molecules may last only seconds or minutes, thus mediating short responses, which are terminated by degradation.

Example: catecholamines (adrenaline, noradrenaline, dopamine) and peptide hormones (range from 3 amino acids to small polypeptides) are produced and stored in secretory vesicles just under the plasma membrane



Cytokines
Include several families of molecules

Expression and activity of cytokines is increased in conditions of tissue stress Minor or moderate tissue ‘stress’ Response to extreme tissue trauma or systemic insult Implicated in numerous pathological conditions.

Signal Transduction

Highly varied responses can be elicited by a particular stimulus on different cells or on the same cell under different conditions. Cellular responses are the result of a network rather than a linear pathway of molecular interactions Many signalling molecules can interact with more than one other type, creating potential for cross-interaction of numerous signalling pathways.

Important features of SIGNAL TRANSDUCTION

What determines the signalling machinery available in a single cell?

Individual cells express numerous receptors, multiple G proteins and many effector proteins. What determines specificity? To elicit the appropriate biological response requires a delicate balance of regulatory mechanisms to control and co-ordinate individual components. Dysregulation of particular components can lead to consequences ranging from cellular malfunction, oncogenic transformation, or death of the organism!

Several mechanisms regulate intensity and duration of a particular signal. It is a balance of on-off switches

Determining the biochemical pathways that are involved in signal transduction

In the past, cellular responses to particular stimuli were worked out by observation, using activators and inhibitors to mimic/disrupt ligand-activated pathways and also by reconstitution assays in which the conditions and concentrations of different components could be investigated in a cell-free environment.

Another approach was to use immunological techniques to work out signalling pathway components

Isolation of particular components not only enabled antibodies to be raised against the molecule, but also enabled characterisation of the primary sequence (particularly for proteins) From the primary sequence, secondary and tertiary structures could be predicted, allowing the identification of structural features (domains) which are important for the particular functions that protein carries out.
Three dimensional structures can be determined accurately from NMR or X-ray diffraction of crystallised molecules (need pure preparation)

Another approach to investigate the importance of the particular peptide sequences which make up structural domains is to synthesise short polypeptides with these sequences and use to disrupt protein-protein interactions.

Yeast two hybrid is a recombinant DNA technique used to investigate protein:protein interactions and can also be used to detect and clone new interacting ‘partners’ for the signaling protein in question (see Lodish 4th ed., Figure 20-29, p 880, for details)

Since the advent of molecular cloning, a large number of different proteins subserving similar functions have been identified. This has enabled new approaches to try and characterise signalling pathways and how they are regulated.

Several potential signalling genes (many of unknown function) have been identified as a consequence of the human genome sequencing project.

It has been predicted that up to 10% of the 30,000-40,000 genes expressed in the human genome are secreted molecules. The challenge is to understand how these prospective signalling molecules function and furthermore, how all these pathways interact! Particularly since abnormalities in signal transduction underlie many different diseases, including cancer and inflammatory conditions.

Lecture notes last updated 16/2/2002

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