Cell signaling

Receptors for cell signals

Cells receive information from their environment through a class of proteins known as receptor ligands. The details of ligand-receptor interactions are fundamental to cell signaling.

As shown in Figure 2 (above, left), Notch acts as a receptor for ligands that are expressed on adjacent cells. While many receptors are cell surface proteins, some are found inside cells. For example, estrogen is a Estrogen receptors inside cells of the uterus can be activated by estrogen that comes from the ovaries, enters the target cells, and binds to estrogen receptors.

Other signaling molecules are unable to permeate the hydrophobic cell membrane due to their hydrophilic nature, so their target receptor is expressed on the membrane. When such signaling molecule activates its receptor, the signal is carried into the cell usually by means of a second messenger such as cAMP.

Signaling pathways

In some cases, receptor activation caused by ligand binding to a receptor is directly coupled to the cell's response to the ligand. For example, the neurotransmitter signal transduction mechanism or pathway.

In the case of Notch-mediated signaling, the signal transduction mechanism can be relatively simple. As shown in Figure 2 (above, left), activation of Notch can cause the Notch protein to be altered by a transcription in the cell nucleus. This causes the responding cell to make different proteins, resulting in an altered pattern of cell behavior. Cell signaling research involves studying the spatial and temporal dynamics of both receptors and the components of signaling pathways that are activated by receptors in various cell types.

A more complex signal transduction pathway is shown in Figure 3. This pathway involves changes of MYC and thus alter gene transcription and, ultimately, cell cycle progression. Many cellular proteins are activated downstream of the growth factor receptors (such as EGFR) that initiate this signal transduction pathway.

Some signaling transduction pathways respond differently depending on the amount of signaling received by the cell. For instance the hedgehog protein activates different genes depending on the amount of hedgehog protein present.

Complex multi-component signal transduction pathways provide opportunities for feedback, signal amplification, and interactions inside one cell between multiple signals and signaling pathways.

Classification of intercellular communication

Main article: Endocrine system#Types of signaling

Within endocrinology (the study of intercellular signalling in animals) and the endocrine system, intercellular signalling is subdivided into the following classifications:

• Endocrine signals are produced by endocrine cells and travel through the blood to reach all parts of the body.
• Paracrine signals target only cells in the vicinity of the emitting cell. Neurotransmitters represent an example.
• Autocrine signals affect only cells that are of the same cell type as the emitting cell. An example for autocrine signals is found in immune cells.
• Juxtacrine signals are transmitted along cell membranes via protein or lipid components integral to the membrane and are capable of affecting either the emitting cell or cells immediately adjacent.

See also

• Cell Signaling Networks
• Molecular Cellular Cognition

References