35 Cell signalling
In order to properly respond to external stimuli, cells have developed complex mechanisms of communication that can receive a message, transfer the information across the plasma membrane, and then produce changes within the cell in response to the message. In multicellular organisms, cells send and receive chemical messages constantly to coordinate the actions of distant organs, tissues, and cells.
While the necessity for cellular communication in larger organisms seems obvious, even single-celled organisms communicate with each other. Yeast cells signal each other to aid in finding other yeast cells for reproduction. Some forms of bacteria coordinate their actions to form large complexes called biofilms or to organise the production of toxins to remove competing organisms. The ability of cells to communicate through chemical signals originated in single cells and was essential for the evolution of multicellular organisms. The efficient and relatively error-free function of communication systems is vital for all life as we know it.
There are two kinds of communication in the world of living cells. Communication between cells is called intercellular signalling, and communication within a cell is called intracellular signalling. An easy way to remember the distinction is by understanding the Latin origin of the prefixes: inter- means ‘between’ (for example, intersecting lines are those that cross each other) and intra- means ‘inside’ (as in intravenous).
Dive deeper
Watch Osmosis by Elsevier. (2020, October 15). Common cell signalling pathways [Youtube, 9:40mins] for an introduction to much of the common terminology used in cell signalling:
Chemical signals are released by signalling cells in the form of small, usually volatile or soluble molecules called ligands. A ligand is a molecule that binds another specific molecule, in some cases, delivering a signal in the process.
Ligands can be thought of as signalling molecules.
Ligands interact with proteins in target cells, which are cells that are affected by chemical signals. These proteins are also called receptors. Ligands and receptors exist in several varieties, however, a specific ligand will have a specific receptor that typically binds only that ligand.
Types of signalling
There are four categories of chemical signalling found in multicellular organisms: paracrine signalling, endocrine signalling, autocrine signalling, and direct signalling across gap junctions (Figure 7.1). The main difference between the different categories of signalling is the distance that the signal travels through the organism to reach the target cell. We should note here that not all cells are affected by the same signals.
Paracrine signalling
Signals that act locally between cells that are close together are called paracrine signals. Paracrine signals move by diffusion through the extracellular matrix. These types of signals usually elicit quick responses that last only a short period of time. In order to keep the response localised, paracrine ligand molecules are normally quickly degraded by enzymes or removed by neighbouring cells. Removing the signals will reestablish the concentration gradient for the signal, allowing them to quickly diffuse through the intracellular space if released again.
For example
One example of paracrine signalling is the transfer of signals across synapses between nerve cells. A nerve cell consists of a cell body, several short, branched extensions called dendrites that receive stimuli, and a long extension called an axon, which transmits signals to other nerve cells or muscle cells. The junction between nerve cells where signal transmission occurs is called a synapse. A synaptic signal is a chemical signal that travels between nerve cells. Signals within the nerve cells are propagated by fast-moving electrical impulses. When these impulses reach the end of the axon, the signal continues on to a dendrite of the next cell by the release of chemical ligands called neurotransmitters from the presynaptic cell (the cell emitting the signal). The neurotransmitters are transported across the very small distances (20–40 nanometres) between nerve cells, which are called chemical synapses (Figure 7.2). The small distance between nerve cells allows the signal to travel quickly; this enables an immediate response, such as, “Take your hand off the stove!”
When the neurotransmitter binds the receptor on the surface of the postsynaptic cell, the electrochemical potential of the target cell changes, and the next electrical impulse is launched. The neurotransmitters that are released into the chemical synapse are degraded quickly or get reabsorbed by the presynaptic cell so that the recipient nerve cell can recover quickly and be prepared to respond rapidly to the next synaptic signal.
Endocrine signalling
Signals from distant cells are called endocrine signals, and they originate from endocrine cells.
In the body, many endocrine cells are located in endocrine glands, such as the thyroid gland, the hypothalamus, and the pituitary gland.
These types of signals usually produce a slower response but have a longer-lasting effect. The ligands released in endocrine signalling are called hormones, signalling molecules that are produced in one part of the body but affect other body regions some distance away. Hormones travel the large distances between endocrine cells and their target cells via the bloodstream, which is a relatively slow way to move throughout the body. Because of their form of transport, hormones become diluted and are present in low concentrations when they act on their target cells. This is different from paracrine signalling, in which local concentrations of ligands can be very high.
Autocrine signalling
Autocrine signals are produced by signalling cells that can also bind to the ligand that is released. This means the signalling cell and the target cell can be the same or a similar cell (the prefix auto- means self, a reminder that the signalling cell sends a signal to itself). This type of signalling often occurs during the early development of an organism to ensure that cells develop into the correct tissues and take on the proper function. Autocrine signalling also regulates pain sensation and inflammatory responses. Further, if a cell is infected with a virus, the cell can signal itself to undergo programmed cell death, killing the virus in the process. In some cases, neighbouring cells of the same type are also influenced by the released ligand. In embryological development, this process of stimulating a group of neighbouring cells may help to direct the differentiation of identical cells into the same cell type, thus ensuring the proper developmental outcome.
Direct signalling across gap junctions
Learning Link: Review the types of cell-cell adhesions including gap junctions.
Gap junctions in animals and plasmodesmata in plants are connections between the plasma membranes of neighbouring cells. These fluid-filled channels allow small signalling molecules, called intracellular mediators, to diffuse between the two cells. Small molecules or ions, such as calcium ions (Ca2+), are able to move between cells, but large molecules like proteins and DNA cannot fit through the channels. The specificity of the channels ensures that the cells remain independent but can quickly and easily transmit signals. The transfer of signalling molecules communicates the current state of the cell that is directly next to the target cell; this allows a group of cells to coordinate their response to a signal that only one of them may have received. In plants, plasmodesmata are ubiquitous, making the entire plant into a giant communication network.
