Biological systems are, in effect, extraordinarily complex machines that rely on a multitude of processes and functions in various diverse systems working together in tandem to create an optimal environment. They rely on complicated and highly evolved signaling systems to provide organs and tissues with information on their correct function. This article will provide an overview of signal transduction and these signaling pathways.
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What is Signal Transduction?
Signal transduction is the term for the process which causes chemical and physical signals to be transmitted within a cell as a series of molecular events. The most common signal transduction process is protein phosphorylation, where proteins are catalyzed by enzymes known as protein kinases. Signal transduction results in a desired cellular response. This can depend on the type of cell itself.
Major pathways include the MAPK/ERK pathway and the cAMP-dependent pathway.
Signal Transduction Pathways
Signal transduction transforms certain stimuli into a biochemical signal which can then elicit a biological response. Such stimuli can include extracellular cues, physical trauma, and intracellular events such as DNA damage. Many intracellular signal relay mechanisms exist in the body including those which govern embryonic development.
Proteins that detect and transduce these stimuli through chemical, physical, or electrical signals are termed receptors. They are the first step in a signal transduction pathway. The signaling chemical (known as a ligand) binds to the receptor molecule, eliciting a cellular or tissue response including a change in the cell’s electrical activity.
Receptors have three main forms of action – relay of signal (signal is sent onward) amplification (the effect of a single ligand is increased) or integration (the signal is incorporated into another pathway.) Receptor proteins are highly specialized.
Once a ligand has bound to the receptor protein it undergoes a conformational change which modifies the shape of the protein and governs how it interacts with molecules around it. One way in which the receptor protein causes the signal/stimuli to be transduced is via the creation of second messengers (including calcium and lipids) which then carry the message to the nucleus or certain specialized organelles within the cell. There are, however, pathways that do not include second messengers.
There is an almost staggering array of signaling pathways in a multicellular organism. The types of receptors and their second messengers can be vastly different from each other.
Signal Transduction Examples
There is an almost staggering array of signaling pathways in a multicellular organism. The types of receptors and their second messengers do have similarities but can also be vastly different from each other. Some examples of signal transduction pathways include vision and touch and hormones.
Vision and Touch Pathways
In this signal transduction pathway, a second messenger is not created. Instead, there is an influx of ions into a cell upon stimulation of the receptor protein. This causes depolarization of the cell membrane. A large enough physical stimulus triggers an action potential through the depolarization of multiple cell membranes at once.
The signal is sent through the cell into the nervous system across a synapse (reaching the brain within microseconds.) To do this, another signal transduction pathway is employed. Specialized receptor proteins at the end of the first cell receive the signal and trigger the release of ligands known as neurotransmitters. These then flow through the intercellular space through gated ion channels into the next nerve cell, and so on, creating more action potentials and traveling through the cells.
Hormone Pathways
Hormones are chemicals the body uses to regulate itself. They control such biological processes as growth and metabolism, carrying out a variety of functions essential to the proper function of the organism.
Endocrine glands such as the thymus and pancreas produce a variety of hormones. Reproductive organs also produce these chemical signals. Signal transduction pathways are intimately involved in the production, release, proliferation, and targeted delivery of hormones to intended cells and tissues. Their transmitted signals are nearly always mediated by a ligand-receptor pathway.
The required hormone, once triggered for release, dissolves into the bloodstream via capillaries in the intracellular space in endocrine tissue and is then carried through the body. They will then bind to specific receptor proteins on target tissues. Once bound, they cause a signal transduction pathway that can have several desired cellular responses. These responses can be different depending upon the tissue or organ targeted.
Signal transduction pathways involved in the regulation of insulin are one such example. If the hormone is needed by muscle cells (to aid in increased physical activity, for example) then the pathway will signal for an increase in glucose transporters in the cell membrane. In the liver, the pathway turns off a key enzyme in glucose production. Thus, the level of glucose within the blood can be effectively regulated by insulin.
Understanding Signal Transduction Pathways
Signal transduction pathways are incredibly complex, diverse, and specific to the type of cell or tissue they target. Understanding them is the focus of several scientists and research projects worldwide.
In doing so, such desired results as manipulating and “hijacking” biochemical responses and processes to provide better-targeted drug delivery and treatment of several diseases and conditions which affect an organism can be achieved.
Scientific disciplines including computational biology, which uses theoretical and data-analytical methods as well as mathematical modeling, machine learning, and complex algorithms to analyze biological systems are being employed to better understand these complex biochemical pathways. Other fields that can be used to elucidate information about signal transduction pathways include systems biology and associated scientific disciplines.
By better understanding the way signal transduction functions, researchers are helping to push the boundaries of medical science and related disciplines ever further.