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!
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.
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.
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.
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.
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 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.
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.
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.
There are several examples of cell signaling, including inflammatory responses, flight-or-fight responses, the action of neurotransmitters, etc.
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.
To respond to a stimulus, a cell must receive a signal and transduce that signal into a response. This process is known as cellular signaling. Cell signaling is crucial for the production of a coordinated response to any external signal. Such cell signaling is mediated by extracellular signaling molecules (first messengers), their corresponding receptors, second messengers, and effector proteins. The stages involved in cell signaling are signal reception, signal transduction, generation of cellular response, and signal termination. Various kinds of signaling molecules exist, including amino acids and their derivatives, steroids, eicosanoids, gases, etc. Cell signaling may be of different types, including endocrine, paracrine, autocrine, and juxtacrine.
Q1. Can two cells carry the receptor for the same signaling molecule?
Ans: Yes, cells can share the same receptor, however, they respond differently to the signal molecule. For example, the β-adrenergic receptor is present in both liver cells and smooth muscle cells. Both are activated by adrenaline. However, in liver cells, adrenaline leads to glycogen breakdown, while in smooth muscles, adrenaline causes relaxation.
Q2. What are the different types of receptors involved in cell signaling?
Ans: G-protein coupled receptors (GPCRs), ligand-gated channels, steroid hormone receptors, and receptor protein-tyrosine kinases (RTKs).
Q3. What are the G-protein coupled receptors and their functions?
Ans: GPCRs are the largest family of cell signaling receptors. They interact with the G proteins. Their functions include hormonal action, neurotransmission, chemotaxis, exocytosis, cell growth and differentiation, blood pressure regulation, etc.
Q4. What are the diseases associated with faulty cellular signaling and communication?
Ans: Inefficient or faulty cell signaling results in cancer, neurodegenerative diseases, autoimmune diseases, etc.
Q5. How is cell-cell contact mediated in bacteria?
Ans: The phenomenon of quorum sensing facilitates cell-to-cell communication among bacterial cells. The cells aggregate on the surface, forming an extracellular matrix around themselves. They then communicate with each other via autoinducers like AHL. The aggregation of communicating cells thus formed is known as a biofilm.
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