Cell Signalling

Intercellular Signaling

• Cells communicate via intercellular signaling (between cells) and intracellular signaling (within a cell).
• Signaling cells release ligands, chemical signals that bind to receptors on target cells.
• Ligands and receptors have specificity—a particular ligand typically binds only to its specific receptor.

Types of Signaling

• Paracrine signaling: Signals diffuse locally through the extracellular matrix, eliciting rapid, short-lived responses. Examples include synaptic signals and neurotransmitters. Signals are quickly degraded to keep the response localized, allowing for the response to quickly change if the signal is released again. A synapse is the junction between nerve cells.
• Endocrine signaling: Long-distance signaling from endocrine cells producing hormones, which have a slower, longer-lasting effect. Hormones travel in the bloodstream.
• Autocrine signaling: Signaling cells can also bind to released ligands, where the signaling cell and target cell can be the same or similar. Crucial in development and regulates processes like pain and inflammation.
• Direct signaling across gap junctions: Intracellular mediators allow small signaling molecules to move between cells through fluid-filled channels in plasma membranes.

Receptors

• Internal receptors: Located in the cytoplasm, bind hydrophobic ligands that cross the plasma membrane. These changes trigger the exposure of a DNA-binding site, influencing gene expression.
• Cell-surface (transmembrane) receptors: Membrane-anchored proteins with three components: an extracellular domain (binding site), a transmembrane domain, and an intracellular domain. Cell-surface receptors are crucial in multicellular organisms and exhibit specificity. Errors in receptors can contribute to health issues like hypertension, asthma, and cancer. Three categories of cell-surface receptors include ion channel-linked, G-protein-linked, and enzyme-linked receptors.

Ion Channel-Linked Receptors

• These receptors bind ligands, opening channels that allow specific ions to pass through cell membranes. Hydrophobic amino acids form the membrane-spanning region of the channel while hydrophilic amino acids line the channel's interior to facilitate ion passage. Ligand binding triggers conformational changes, affecting ion flow.

G-Protein-Linked Receptors

• Involved in various cellular processes and are susceptible to interference by pathogenic bacteria like those causing pertussis, botulism, and cholera.

Enzyme-Linked Receptors

• These receptors have intracellular domains associated with enzymes. The enzymatic activity is triggered by ligand binding, initiating intracellular signaling cascades. Examples include tyrosine kinase receptors like HER2, a receptor involved in cancer. Drugs like lapatinib can reduce tumor growth by inhibiting HER2 receptor activation. Insulin also interacts with an enzyme linked receptor and triggers responses like glycogen synthesis and glucose uptake.

Ligands and Signal Transmission in Multicellular Organisms

• Small Hydrophobic Ligands: Directly pass through the plasma membrane and interact with internal receptors. Examples include steroids (estradiol, testosterone, cholesterol), thyroid hormones, and vitamin D.
• Water-soluble Ligands: Cannot pass through the membrane due to polarity or size and bind to cell-surface receptors.
• Other Ligands: Nitric oxide diffuses directly through the plasma membrane and interacts with receptors in smooth muscle, inducing tissue relaxation.

Receptor Involvement in Health

• Receptor-related problems like downregulation of insulin receptors or a loss of receptor sensitivity, can lead to issues like insulin resistance.

Viral Interactions with Host Cells

• Viruses rely on host cells to reproduce and often target cell-surface receptors to enter. Differences in receptor types across species can limit viral spread between animal species. Viral mutations allow viruses to bind to receptors in new hosts, enabling rapid spread.

Test your knowledge on intercellular signaling methods and their mechanisms. This quiz covers various types of signaling, including paracrine, endocrine, and autocrine signaling. Dive into the specificity of ligands and receptors and their roles in cellular communication.