Wednesday, February 20, 2013

Cell-Cell Signaling

Have you ever wondered, how do cells talk with each other or how do they communicate? How do they come to know what has to be done if you get hurt or if you are hungry or thirsty? Isn't it very interesting to know that all types of cell receive information and respond to signals from their environment. But you might be wondering, how? The answer is through ‘Signaling Molecules’ which play a lead role in this process of cell-cell communication. Now, you might be thinking, what are these signaling molecules? Where do they come from? How do they work? Ok..lets understand all about signaling molecules. These signaling molecules are basically chemicals (like nitric oxide etc.) or proteins (hormones etc.) which are secreted or expressed on the surface of the cell. They then bind to receptors which are present on the other cell (these cells are called target cell)  or sometimes even present on the same cell; thereby coordinating the functions of various cells. The binding of these signaling molecules on the receptor creates a series of reactions that regulates various methods/systems like movement, metabolism, survival, differentiation etc. The breakdown of these pathways has resulted in various types of cancer and hence has become a very interesting field to study.

Modes of Cell-Cell Signaling:
Signaling by Direct Cell-to-Cell Interaction
Cell signaling can take place by direct cell-to-cell interaction (as can be seen in the adjacent figure) where, on the surface of one cell, is the signaling molecule that binds directly to receptor, which is present on the surface of the other cell. The cell signaling/ cell-cell communication can also take place by the action of various signaling molecules. As already mentioned above, various cell receptors are present on the target cells which bind to signaling molecules.
The different types/modes of signaling by secreted molecules are mainly of three types:
Diagrammatic Representation of Endocrine Signaling
1. Endocrine Signaling: Is it difficult to remember these different types of signaling with their functions? Don't worry! I will try to make it easy for you - split the word and understand the  meaning of individual word, then its easy to remember these words with their functions. So, lets understand the meaning of endocrine. The word “endocrine” originates from two words, 'endo' and 'crine'. The former word means 'within' and the suffix 'crine' means 'to secrete' or 'separate' Thus, the word endocrine means ‘secrete from within’. In this type of signaling, the signaling molecules are secreted from within the endocrine glands; they travel through the circulation system and act on/ bind to receptor molecules on the target cell (as can be seen in the above figure). The signaling molecules in this type of signaling are ‘hormones'. Hormones are secreted by specialized endocrine cells of endocrine glands which act on target cells which are at distant and reach via the circulation system. An example of endocrine signaling: The hormone, insulin, is secreted by pancreas which then travel through the circulatory system which has an effect of stimulating muscle cell or adipose cells.
Diagrammatic Representation of Paracrine Signaling
2. Paracrine Signaling: 'Para' means 'nearby' and 'crine' means ' to secrete'. Thus, 'paracrine' word means 'secretion nearby'. Thus, in this type of signaling, the molecules (signaling molecule) released by one cell acts on or bind to the receptor on the nearby neighbouring cell. A very common example of paracrine signaling is the neurotrasmitters. These are present at the synapse  or junction of neuron cells and help in transmitting the signals. So, neurotransmitters are released by one neuron cell and they act on nearby neuron cell to transmit the signal.

Diagrammatic Representation of Autocrine Signaling
3. Autocrine Signaling: 'Auto' means 'self' and 'crine' means 'to secrete'. Thus, 'autocrine' means 'secrete to itself' Thus, in this type of signaling, the molecules (receptor molecules) are secreted from the cell and act on the same cell. In other words, some cells respond to signaling molecules that they themselves produce.
For example, the cytokine, Interleukin-1 (IL-1) in monocytes. The interelukin is produced in monocytes response to some external stimuli and it binds to receptor present on the surface of its own cell. 

I hope now you will remember these different types of signaling.