Cell Communication

1
Cell Communication

• Cells need to communicate with one another,
  whether they are located close to each other or
  far apart.
• Extracellular signaling molecules regulate
  interactions between cells.
• The basic mechanism requires a ligand (signaling
  molecule) to interact with its receptor so as to
  convert the extracellular signal to an
  intracellular signal. This process is called
  signal transduction and can occur in several
  forms.

2
Types of Cell Signaling

• If a signal is needed to communicate with the
  entire organism the signaling molecule is
  secreted into the bloodstream.
• Endocrine signaling requires a cell to synthesize
  and secrete a hormone into the circulatory
  system.
• That hormone is then recognized by a specific
  target cell protein (receptor) either on the
  plasma membrane or within the cytoplasm.

3
Types of Cell Signaling

• In other situations the signal is required to act
  locally.
• Paracrine signaling molecules (local mediators)
  can be released by a neighboring cell, diffuse
  locally through the ECM, and stimulate a close
  target cell.
• Growth and differentiation factors are thought to
  act primarily as paracrine signaling molecules.

4
Types of Cell Signaling

• A third form of communication is neuronal
  signaling.
• This type of signal transduction can occur over
  long distances, however the delivery is by way of
  long cellular processes called axons.
• Neuronal signals can act on target cells or on
  other neuronal cells.
• The signal travels through the axon as an
  electrical impulse that upon reaching the axon
  terminus it is converted into a chemical signal
  called a neurotransmitter.

5
Types of Cell Signaling

• The fourth style of signal transduction is the
  most short-range of all. It typically does not
  require the release of a secreted molecule.
• Contact-dependent signaling requires the
  transduction to be completed when the signaling
  molecule anchored in the PM of the signaling cell
  to bind to the receptor molecule embedded in the
  PM of the target cell.
• Contact-dependent signaling can also occur in the
  form of a cell interacting with the ECM.

6
Cellular Responses to Signal Molecules

• Each cell is able to respond to a limited set of
  signals due to its specialized function as well
  as its limited types of receptors.
• Additional, each cell responds to a signal
  molecule differently.
• In this case, the neurotransmitter, acetylcholine
  can stimulate one type of muscle cell to contract
  (skeletal) or inhibit contraction in another
  (cardiac). Acetylcholine can also stimulate
  certain cells to secrete the contents of their
  secretory vesicles.
• Similar receptors can activate different
  intracellular signaling pathways, or the ligand
  binds to different receptors.

7
Cell Responses to Extracellular Signals

• Often, cells require multiple signals to elicit
  the proper response.
• The possible cellular responses include
• Survival and/or normal cellular functioning.
• Proliferation
• Differentiation
• Cell death
• The different combinations of signals allows a
  relatively small number of signaling molecules to
  be used to control the complex behavior of a cell.

8
Intracellular Signaling Cascades

• Signal transduction begins when the ligand or
  signaling molecule binds to its receptor.
• This binding event elicits an intracellular
  signaling mechanism that passes the signal from
  one intracellular signaling molecule to another
  until the final outcome or response of the cell
  is achieved.
• This signaling cascade allows for the
  amplification, modulation, and distribution of
  the signal inside the cell.

9
Intracellular Signaling Cascades

• Features of signal cascades include
• 1. They can physically transfer the signal from
  where its received, either on the plasma
  membrane or in the cytoplasm, to the cell
  machinery where the response occurs.
• 2. They can transform the signal into the
  molecular form that is able to stimulate that
  response.
• 3. They can amplify the signal received, making
  it stronger, so that fewer signaling molecules
  are needed to produce the response.
• 4. They can distribute the signal to several
  cellular locations in order to influence several
  processes simultaneously.
• 5. Each step in the signaling cascade is open to
  modification or alteration by other factors
  whether they are outside or inside the cell.

10
Types of Signaling Molecules

• Most signaling molecules are hydrophilic and
  therefore unable to cross the plasma membrane.
  They instead bind to cell-surface receptors which
  generate intracellular signals inside the target
  cell.
• In contrast, some small hydrophobic signaling
  molecules can and do diffuse across the PM and
  bind to receptors located within the cytosol or
  nucleus of the target cell.

11
Hydrophobic Hormones
12
Steroid Hormone Signaling Pathway

• Steroid hormones, being hydrophobic molecules are
  able to diffuse across the PM where they can
  encounter their receptor located within the
  cytoplasm.
• The ligand-hormone complex can then move into the
  nucleus where it acts as a gene regulatory
  complex by binding to a specific regulatory
  sequence in the DNA and activating gene
  transcription.
• Other steroid hormone receptors are already
  located in the nucleus.

13
Cell-Surface Receptors

• Most cell-surface receptors belong to one of
  three large families.
• Ion-channel-linked receptors - when a ligand
  binds to this receptor it opens or closes an ion
  channel.
• G-protein-linked receptors - when its ligand
  binds, this receptor it activates a G-protein
  that initiates a cascade of effects.
• Enzyme-linked receptors - when activated by
  ligand binding, this receptor switches on an
  enzyme on its cytoplasmic side and generates a
  downstream cascade of signals.