Cell signaling

Cell signaling is the process through which cells communicate to properly respond to changes in their immediate environment. Cell signaling allows cells to receive and react to messages from outside their own walls, as well as to transmit those messages to other cells. The process of cell signaling is made possible by special chemical signals that trigger a response when detected by cells. Because it enables cells to perceive and respond to environmental changes, cell signaling is a crucial biological pathway that makes important processes such as fetal development, tissue repair, and immunity possible. As a result, cell signaling is one of the most critical functions in the development and survival of all multicellular organisms. Problems with cell signaling can also lead to disease. Defects in cell signaling pathways can quickly give rise to diseases such as cancer and diabetes, or to autoimmune conditions like rheumatoid arthritis and multiple sclerosis.

Background

The modern scientific understanding of cell signaling is a relatively recent development. In fact, the science of cell signaling was not really understood at all until the twentieth century. Physicians attempted to unlock the secrets of the body's inner workings from the earliest days of biological studies. For centuries, physicians believed that health was determined by the balance of substances called humors in the body. The first significant shift in this belief as it pertains to cell signaling came in the early twentieth century thanks to the efforts of physiologist J.N. Langley and physician Paul Ehrlich. Both men independently developed the idea that there was a specific entity that agonists, or special agents that stimulate certain biological responses, react with to trigger biological effects. Langley conducted studies with various types of muscles and drugs such as nicotine. He found that nicotine and other drugs acted with substances found within the muscle tissue rather than with the nerves that control the muscles. He referred to this substance as receptive substance. Ehrlich described the interactions that occur between molecules called ligands and the larger cell parts called receptors that bind them.

In the years that followed, scientists expended a great deal of time and effort on studying receptors and what they do. Eventually, they came to learn that receptors played a pivotal part in what came to be known as cell signaling, or the process through which cells communicate and biological effects are triggered. Scientists also learned about the presence of special molecules called second messengers and the role they play in cell signaling. First discovered by American physician Earl Sutherland, second messengers are molecules inside cells that transmit signals from a receptor to a target. Continued research led to further breakthroughs as time went on. All these breakthroughs helped scientists better understand cell signaling, how it works, and the role it plays in human health. As a result, scientists in the twenty-first century recognize that cell signaling is an essential process for all multicellular organisms and a major biological function without which complex life could not exist. In 2024, a team of scientists from MIT, Harvard University, and Yale University announced that they had completed a comprehensive atlas of human tyrosine kinases. Referred to as the "Rosetta Stone" of cell signaling, this achievement could lead to numerous medical advancements in the future.

Overview

Cells communicate with one another with the help of chemical signals. In most cases, these chemical signals are secreted into extracellular space by one cell and float to other surrounding cells. When a cell secretes signaling molecules, which are also called ligands, it acts as a sending cell. Ligands secreted by sending cells can only be detected by target cells that possess the correct receptor for the molecules in question. When a signal reaches a target cell, a reaction occurs between the ligand and the intended receptor within the cell. Specifically, the ligand alters either the shape or activity of the receptor. This, in turn, sets off a cascade of events within the cell during which the ligand's message is relayed by a series of chemical messengers until some sort of change is finally triggered. As a result, cellular signaling can be described as the process through which an intercellular signal sent between different cells is converted into an intracellular signal within a single cell that triggers some sort of response.

There are four basic types of cell signaling that occur in multicellular organisms. These include paracrine signaling, autocrine signaling, endocrine signaling, and signaling by direct contact. These different forms of cell signaling are primarily distinguished from one another by the distance that ligands travel to reach the target cell.

Paracrine signaling is cell signaling that takes place over short distances. Neighboring cells rely on paracrine signaling to coordinate activities so that they can all work together to tackle certain tasks. Autocrine signaling occurs when a cell sends signals to itself. During fetal development, autocrine signaling helps cells to take on and maintain their individual identities. Autocrine signaling is also believed to play a critical role in metastasis, which is the process through which cancer spreads from its point of origin to other parts of the body.

Endocrine is the longest-distance form of cell signaling. In endocrine signaling, sending cells release signals into the bloodstream so that they can ultimately reach target cells in distant parts of the body. The signals involved in endocrine signaling are known as hormones. Hormones are released by endocrine glands such as the pituitary and the thyroid. When signaling occurs through direct cell-to-cell contact, special signaling molecules called intracellular mediators are delivered from a sending cell to a target cell through tiny connective channels called gap junctions. Through this type of signaling, a cell can share important information about its current state with its connected neighbor cell and receive the same sort of information from its neighbor. In practice, this allows the cells in question to coordinate a response to a broader signal, even if only one of them actually receives it.

There is also another type of direct signaling, in which cells with complementary surface proteins bind together to transmit a signal. This kind of cell signaling plays an important part in the immune system, where it allows immune cells to tell the difference between the body's own cells and cells that have been infected by pathogens.

Bibliography

Bradshaw, Ralph A., and Edward A. Dennis, editors. Handbook of Cell Signaling. Academic P, 2010.

"Cell Signaling." Scitable, www.nature.com/scitable/topicpage/cell-signaling-14047077. Accessed 7 Nov. 2024.

"Cell Signaling Definition." Biology Dictionary, 5 Jan. 2021, biologydictionary.net/cell-signaling. Accessed 7 Nov. 2024.

Hancock, John T. Cell Signaling. Oxford UP, 2017.

"Introduction to Cell Signaling." Khan Academy, www.khanacademy.org/science/biology/cell-signaling/mechanisms-of-cell-signaling/a/introduction-to-cell-signaling. Accessed 7 Nov. 2024..

Lim, Wendell, Bruce Mayer, and Tony Pawson. Cell Signaling: Principles and Mechanisms. Garland Science, 2015.

Scudellari, Megan. "'Rosetta Stone' of Cell Signaling Could Expedite Precision Cancer Medicine." MIT News, 3 June 2024, news.mit.edu/2024/rosetta-stone-cell-signaling-could-expedite-precision-cancer-medicine-0603. Accessed 7 Nov. 2024.

"The Inside Story of Cell Communication." Learn. Genetics, learn.genetics.utah.edu/content/cells/insidestory. Accessed 7 Nov. 2024.