cell signals

Autocrine

  • cells signals to itself

Endocrine signalling.

  • Signal to a far away cell

Paracrine signalling

  • cells signals to a lose by cell.

Conceptual basis

  • to make multicellular organism cells must communicate.
    • this communication is mediated by extracellular signal molecules.
    • Sophisticated mechanism control which signal molecules are released from specific cell type , at which time and concentration they are secreted, and how these signals are interpreted by the target cells.
    • some signalling molecules act over long distances, some act only on immediate neighbour cells.
    • most cells in higher organism are both emitters and receivers of signals.

Budding yeast cells responding to mating factor

  • the cells are normally spherical.
  • in response to mating factor secreted by neighbouring yeast cells,
  • they put out protrusions towards the source of the factor in preparation for mating.

Basic terms
Transcription factor - A ligand binds to its receptor.

  • allowing for the complex to move into the cell nucleus. , there, the complex acts as a transcription factor.

  • Different ligands can activate/ inhibit transcription.


  • signalling molecules (ligands)

  • bind to (ligate)
  • Specific receptors.
  • Binding to a ligand to receptor outside the cells. - Receptor is integrated in the cell membrane.
  • Intracellular signalling cascade gets activated. - these in return active effector proteins which change metabolism., gene expression, shape/movement of cell.

Basic terms


intercellular signalling

  • how one cell communicates with others cells within an organism.
  • Direct physical contact.
  • Nearby
  • Distant tissue.

Intracellular signalling

  • how signals from receptors are transmitted within a cell.
  • how ligation of a receptor causes changes to cell behaviour.

Conceptual basis.

  • Extracellular signal molecules (ligands) can bind to (ligate) either cell-surface receptors or intracellular receptors.
    • most signal molecules are hydrophilic and are therefore unable to cross the plasma membrane directly: instead, they bind to cell surface receptors, which in turn generates one or more signals within the targeted cell.
    • some small signal molecules, by contrast, diffuse across the plasma membrane and bind to receptors inside the target cell either in the cytosol or in the nucleus.
    • many of the small signal molecules are hydrophobic and nearly insoluble in aqueous solution: they are therefore transported in the bloodstream. and other extracellular fluids after binding to carrier proteins, which the disassociate from before entering targeted cells.

Cell surface or intracellular receptor signalling -


  • Hydrophilic signal activates


    cell-surface receptor-> intracellular signalling cascade is activated.


  • Hydrophobic signal detaches


    from carrier protein, passes through cell membrane and activates receptors in the nucleus.

four forms of intercellular signalling


Contact- dependent.
immune cells: Antigen presentation.

  • Paracrine
  • Budding yeast cells
    Synaptic
  • Nerve cells.
    Endocrine
  • Hormones.

Synaptic vs Endocrine


Endocrine signalling

  • Transport of signals through blood stream
    Synaptic signalling
  • Transports of signal vai axons in nerve cells.

Effects of cell signalling

  • Survive
  • Grow and survive
  • Differentiate
  • apoptosis ( controlled cell death)

cell signals

Receptor classes.


Nuclear receptors

  • Found with the cell nucleus.

Cell surface.
Ions channels coupled

  • G- protein coupled receptors
  • enzyme coupled receptors.

Nuclear receptor

  • Binding of ligand to receptor causes a conformational change, coactivator
    protein can now bind ->transcription

Example: Steroid hormones signalling

  • Activation of primary response genes
    induces primary response proteins.
    The primary response proteins a) shut
    off primary response genes and b) activate secondary response genes -> secondary
    response proteins are made.

Three surface receptor classes


ion-channel-linked receptors
converts chemical signals to electrical signals.
Example Gap junctions

G-protein-linked receptor

  • Signal transduction involves the production of 2nd messengers.
    Example chemokine receptors, adrenergic receptors.

Enzyme linked receptors
Protein kinase receptors
Example is insulin

Ion channel coupled receptors
- Reside within excitable tissues.

  • Example neuromuscular junction--> nerve impulse leads to release of acetylcholine which in turn activates cation channels.

How is signalling information encoded - Signal acts as molecular switch to
change the conformation of a protein active/inactive.

  • this work through the addition/removal of a phosphate group.
  • P gets added to protein.
  • GDP gets substituted by GTP

Not all amino acids can be phosphorylated:

  • Serine ( unchanged)
  • Threonine (unchanged)

  • Tyrosine (hydrophobic).


  • Phospho-serine


    -phospho-threonine

  • phospho-tyrosine
    all negative and polar.

Effects of phosphorylation.

  • change of charge results in conformation change (shape is different)
  • phosphorylation is the attachment of a phosphate group to a molecule or an ion.

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