Biochemical cascade

Cells rely on a full set of working machinery to survive. In complex multicellular organisms, they must communicate and cooperate to sustain the whole body. This communication triggers internal signaling cascades—also called signal transduction pathways—that control specific cellular functions. A cascade begins when an external first messenger binds to a receptor, either on the cell membrane or inside the nucleus. This binding starts intracellular signals. The resulting complex produces or releases second messengers, which integrate, adapt, and amplify the signal by activating molecular targets. Those targets then trigger effectors that produce the desired cellular response.

Signal transduction works through the activation of specific receptors and the subsequent production or release of second messengers, such as Ca²⁺ or cAMP. These molecules act as signal transducers, setting off intracellular cascades and amplifying the original signal. Two main mechanisms exist: via nuclear receptors or via transmembrane receptors. In the first, the first messenger crosses the cell membrane—possible because these ligands (mostly hormones) are lipophilic—and binds to intracellular receptors in the nucleus or cytosol. These receptors then act as transcription factors, directly regulating gene expression. In the second mechanism, the first messenger binds to the extracellular part of a transmembrane receptor, activating it. These receptors can have intrinsic catalytic activity, be coupled to effector enzymes, or be linked to ion channels. Consequently, there are four main types of transmembrane receptors: G protein-coupled receptors (GPCRs), tyrosine kinase receptors (RTKs), serine/threonine kinase receptors (RSTKs), and ligand-gated ion channels (LGICs).

Second messengers fall into three classes. Hydrophilic or cytosolic messengers—such as cAMP, cGMP, IP₃, Ca²⁺, cADPR, and S1P—are water-soluble and stay in the cytosol. They mainly target protein kinases like PKA and PKG, leading to phosphorylation-driven responses. Hydrophobic or membrane-associated messengers—including PIP₃, DAG, phosphatidic acid, arachidonic acid, and ceramide—are water-insoluble and stay near membranes, where they bind to membrane-associated effector proteins. They regulate kinases, phosphatases, G protein-related factors, and transcription factors. Gaseous messengers—nitric oxide and carbon monoxide—can spread through cell membranes and the cytosol. Both activate cGMP and can act independently or together.

The cellular response to a signaling cascade involves either changing the expression of effector genes or activating or inhibiting specific proteins. Most responses from membrane-receptor binding involve phosphorylation or dephosphorylation, which quickly activates or inhibits proteins already present. In contrast, turning genes on or off requires transcription factors to bind to regulatory sequences. These factors are often activated by primary messengers acting as nuclear receptors. Secondary messengers like DAG or Ca²⁺ can also influence gene expression through transcription factors. This gene-based response is slower because it requires transcription and the production of new proteins, which then act on a target—another protein or a gene.

Many important enzymatic and signal transduction cascades participate in metabolic pathways or signaling networks, with enzymes usually catalyzing the reactions. One example is the tissue factor pathway in the coagulation cascade of secondary hemostasis, which leads to fibrin formation and starts blood clotting. Another is the sonic hedgehog signaling pathway, a key regulator of embryonic development found in all bilaterians. Signaling proteins in this pathway provide cells with instructions for proper embryo development. When the pathway malfunctions, it can cause diseases like basal cell carcinoma. Recent research indicates that hedgehog signaling also helps regulate adult stem cells involved in maintaining and regenerating adult tissues. The pathway has been linked to the development of some cancers, and several pharmaceutical companies are actively developing drugs that target hedgehog signaling to treat these diseases.