Cell Signaling and Apoptosis
Types of Signaling
• A ligand is a molecule that has specificity for a given receptor, and the binding of a ligand to the
... [Show More] given
receptor causes a change.
• There is a wide array of cell ligands: hormones, peptides, sugars, and even small, hydrophobic molecules.
• Regardless of its type, a ligand is always specific for the target receptor. This ligand-to-receptor specificity
helps to regulate responses in the target cell while not impacting non-target cells that may be in the same
location.
• When the sending cell releases a ligand and the target cell receives the “message”, the process is
called intercellular communication (inter = between cells).
• There are five main types of intercellular signaling: (1) paracrine signaling, (2) synaptic signaling, (3)
autocrine signaling, (4) endocrine signaling, and (5) direct cell-to-cell signaling.
1. Paracrine signaling is a type of local range signaling, meaning the signal travels over a short distance.
In paracrine signaling, a signaling cell will release vesicles containing the signaling ligand, and the ligand
will then bind to nearby target cells. This type of signaling occurs in many types of cells and tissues, and
it is of great importance during development and for the secretion of growth factors.
2. Synaptic signaling is another form of local range signaling, but it occurs across a synapse
• A synapse is a small gap between the signaling cell (also called the pre-synaptic cell) and the target cell
(also called the post-synaptic cell).
• Many times, the pre-synaptic cell is a neuron containing molecules to be released, and the post-synaptic
cell may be another neuron or a muscle cell.
• An electrical signal is propagated down the pre-synaptic cell and stimulates the release
of neurotransmitters, a class of ligand molecules that are released from neurons, that are packaged in
vesicles. Neurotransmitters diffuse across the synapse and bind to the target cell.
• Neurotransmitters have a rapid action and bind quickly to their target cell to cause a change in the target
cell.
3. Autocrine signaling is a type of signaling that occurs when a cell acts as both the sender and receiver of a
signal. Essentially, the cell signals itself, releasing a ligand that binds to receptors on its own surface.
Autocrine signaling is common to immune cells and to the progression and metastasis (spreading) of cancer
cells.
4. Endocrine signaling is used to transmit ligands to their target cells over long distances. The circulatory
system serves as the transport medium to move endocrine ligands around the body. Commonly,
• Hormones are the type of ligand released and transported in this type of signaling.
• Hormones are often involved in regulating homeostatic mechanisms, including blood pressure and energy
metabolism.
• Note that hormones will not bind to just any cell they meet; the target cells must have receptors that are
specific for the hormone ligands to bind.
5. Direct cell-to-cell signaling involves contact between the sending cell and the target cell. An example of
direct cell-to-cell signaling can be seen with gap junctions. Recall that gap junctions are tiny channels or
pores located between cells.
• These pores allow for small, intracellular (intra = within cells) signaling molecules to easily pass from one
cell to an adjacent cell. An example of common intracellular molecules is calcium ions.
• Transferring small molecules like calcium from cell to cell allows for connected cells to coordinate a
response even though only one cell may have received the ligand.
• Cell-to-cell recognition signaling is another type of direct cell-to-cell signaling that occurs when two cells
have a physical interaction that occurs on the surface of both cells.
• This particular type of direct cell-to-cell signaling is important for regulation of immune system responses.
Communication: A Three-Step Process
• For a cell receiving an incoming message, there is a three-step process that usually occurs: reception,
transduction, and response
1.Reception occurs through receptors, which are usually anchored in the cell membrane. A chemical
message is received by the target cell when a signaling molecule binds to a membrane receptor or to a
receptor inside the cell, called an intracellular receptor. The binding of the molecule causes a change within
the cell that sets off a series of communication pathways.
2. Transduction occurs when the signaling molecule causes an intracellular change. Transduction may occur
as a single step, but most often it occurs as a series of sequential steps collectively known as a signal
transduction pathway.
• When there are several molecules involved in the pathway, the molecules are called relay molecules.
3. A response is triggered within the target cell after the signal has been successfully transduced.
• Responses can include activation of genes within the nucleus, changes to the cytoskeleton, release of
vesicles containing hormones, or even controlled cell death
Reception
• Most receptors are located on the outside of [Show Less]