Introduction
Imagine a deer fleeing for its life as it is being chased to death by a predatory cheetah. The deer runs fast and during this its breathing rate increases, and heart pounds furiously in anxiousness. Its legs move the fastest they possibly can. Such intense physiological responses to an external threat (stimulus) are a part of the deer’s flight-or-fight response, wherein the nervous system, muscular system, and skeletal system work together to respond aptly. Such and other responses including wound healing, defense against pathogens, etc., are made possible only through cellular signaling and communication. How does the body know how and when to react? What is cell signaling? How is cellular communication mediated? Let’s find out!
Meaning of Cell Signaling
- Cells communicate with each other to work together and coordinate cellular activities and bodily functions.
- They need to receive and respond to the signals from their surroundings.
- The whole process wherein a cell receives a signal, processes it, and transmits it is known as cell signaling. Cell signaling is sometimes also referred to as cell communication.
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Features of Cell Signaling
- Cell communication or signaling is mediated by special molecules called extracellular messenger (or signal) molecules.
- Examples of extracellular signaling molecules (aka ligands) include hormones, steroids, neurotransmitters, glycoproteins, amino acids, gases, etc.
- Cell signaling may be long-range, wherein the signal molecules are secreted from one cell and the target is at a distant site, or short-range wherein the target cell is located nearby the producer cell.
- Once the ligand (i.e., the signal molecule) binds to its receptor, the latter undergoes conformational changes which lead to a relay reaction across the cell.
- Signaling molecules are of two kinds: secretory molecules, and membrane-bound molecules.
- The receptors involved in cell signaling are also of two types- the cell-surface receptors and the intracellular receptors.
- The cell-surface receptors convert extracellular ligand binding into an intracellular signal.
Stages in Cell Signalling
Cell signaling involves the following stages −
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- Signal reception: The signal molecule binds to the receptor.
- Signal transduction: In this stage, the signal converts to a form that can bring about a cellular response.
- This stage may occur as a single step or a series of steps referred to as the signal transduction pathway.
- The molecules involved are also known as relay molecules.
- This series of reactions that occurs is also known as a cascade.
- Often, molecules called second messengers are generated in signal transduction. These molecules enhance the response of the cell. Examples of second messengers include cAMP, Ca2+, diacylglycerol, etc.
- Cellular response: Depending on the message and the kind of cell that’s receiving the signal, the response may involve:
- Changes in gene expression
- Enzyme catalysis
- Protein synthesis
- Reconfiguration of cytoskeleton
- Changes in membrane permeability
- Transcription/replication
- Transcription/replication
- Signal termination: In signal termination, the extracellular signaling molecule will be eliminated, either by the action of extracellular enzymes or by internalization and degradation of the ligand-receptor complex.
Types of Cell Signalling
Cell signaling is of the following types, depending upon the nature of the messenger molecule, the origin of the signal, and the location of the action.
Endocrine Signaling
- This type of signaling is mediated by signaling molecules known as hormones, generated from endocrine cells.
- These hormones operate on cells that are located in distant sites within the body. Hence, endocrine signaling mediates long-range communication.
- The endocrine signaling molecules, i.e., the hormones, are transported via the bloodstream.
Paracrine Signalling
- Paracrine signaling is different from endocrine signaling in that the former affects target cells that are in proximity.
- Hence, the messenger of paracrine signaling operates over short distances, therefore paracrine signaling is said to be local signaling or short-range signaling.
- Examples of paracrine signaling include synaptic signaling, inflammatory responses.
Autocrine Signalling
- As suggested by the name, in autocrine signaling, the signaling molecules produce the effect on the same cell that produces the molecules.
- The producer cell also expresses the receptor for its messenger.
- Such cells will stimulate or inhibit themselves.
- Such cells respond to an extracellular messenger molecule only if the cells carry the corresponding receptor for the extracellular molecule.
- Examples of autocrine signaling include programmed cell deaths.
Juxtacrine Signalling
- In juxtracrine signaling, the signaling molecules are expressed on the cell surface, and not secreted. These cells are then capable of interacting with the corresponding receptors of the adjacent cells.
- Juxtacrine signaling is also referred to as contact-dependent signaling.
Examples of Cell Signaling
There are several examples of cell signaling, including inflammatory responses, flight-or-fight responses, the action of neurotransmitters, etc.
Cell to Cell Contact Signalling
- More often than not, cells in multicellular organisms need to communicate the message that one of them receives, with other cells too.
- This is known as cell-cell contact signaling, mediated by cellular connections known as gap junctions (animals) and plasmodesmata (plants).
- These channels facilitate the transmission of the signaling molecules between the two cells. These molecules are known as intracellular mediators.
- Such cell-cell contact facilitates coordination of the two cells, further promoting cell communication and response to a particular stimulus that only one cell receives.