Organisms exhibit a range of adaptive behaviors in response to changes in the environment. Even unicellular organisms respond to various physical and chemical stimuli. Multicellular organisms, including animals and plants, have a well-developed intercellular signal transduction system, and networks of cells come together to establish these organisms. Intracellular signal transduction is activated when intercellular signals reach cells.
Figure 9-1A shows the basic mechanism of intracellular signal transduction. Cells have many receptor proteins on their plasma membrane surface, and molecules that bind specifically to these receptors are called signaling molecules. When bound with signaling molecules, receptors go through structural changes (such as becoming dimers) and are activated, which then changes the shape, movement and functions of the cell by activating intracellular signal transduction proteins, and regulates gene expression through the relocation of intracellular signaling molecules to the nucleus.
While a large number of physical and chemical stimuli exist in the environment, organisms respond only to particular stimuli.
This is due to the signal detection and communication mechanisms of their cells, and in particular due to the limited number of receptor proteins. Analyzing the genes and proteins involved in signal transduction will therefore enable a better understanding of the basic mechanisms of cellular regulation.
In multicellular organisms, intercellular exchanges of signals are performed in various ways, as shown in Figure. 9-1B. Intercellular signaling molecules include hormones, growth factors and cytokines. Hormones are generally signaling molecules that act on distant target cells. Endocrine disrupters, a.k.a. environmental hormones, are exogenous chemicals that exhibit hormone-like effects by acting on receptors. The molecules involved in signal transduction between cells are called first messengers, while the diffusible molecules engaged in intracellular signal transduction in response to first messengers are known as second messengers (e.g., Ca2+ and cAMP).
As exceptional signalers, gaseous molecules (such as nitric oxide) and fat-soluble molecules (such as steroid hormones) enter the cytoplasm by crossing the plasma membrane; the receptors are therefore located in the cell or the nucleus. Gaseous molecular ethylene (a plant hormone) reacts with the receptors on the plasma membrane. Light penetrates the cytoplasm and is detected by specific pigment-carrying receptors.
Fig. 9-1. Basic mechanism of signal transduction
A) Basic mechanism of intracellular signal transduction,
B) Mode of intercellular signal transduction: (a) autocrine mode: cells are activated by the signaling molecules they secrete, (b) paracrine mode: cells activate surrounding cells, (c) endocrine mode: signaling molecules activate distant cells by traveling through blood vessels and other pathways, (d) cell-contact mode: cells are activated through contact with their membrane proteins, and (e) nerve mode: part of a cell protrudes and forms a synapse.