Receptors and target cells

Receptor life cycle and regulation

The activity of receptor structures is not constant. It is dynamic and determined by two key parameters: the total number (density) on the cell membrane or inside the cell, as well as the degree of affinity for a specific ligand.

The life cycle of receptors is continuous. The number of these structures in target cells is constantly changing as they go through several stages:

• Continuous synthesis of new protein molecules.
• Temporary inactivation during the performance of their functions.
• Degradation of utilized structures.
• Reactivation and return to working condition.

There are two main mechanisms for regulating their activity:

1. Ligand concentration dependence. The target cell adapts to the level of the incoming signal. If a biologically active substance (e.g., a hormone or neurotransmitter) is insufficient, a compensatory reaction occurs: the number of active receptors increases, leading to increased tissue sensitivity. Conversely, in the event of a hormone excess, the number of active structures or their sensitivity naturally decreases.
2. Internalization. This mechanism is most characteristic of receptors interacting with protein-peptide hormones and catecholamines. Once the ligand binds to the receptor, the resulting complex is internalized into the cell. The consequence of such internalization is the inactivation of the receptor structure.

Interaction with immune cells: T-killers

Target cells can undergo targeted destruction during immune responses. T-killers use an arsenal of cytotoxic mechanisms to eliminate cells, combining the processes of apoptosis (programmed cell death) and necrosis.

• Enzyme penetration (perforin-granzyme system). The T-killer releases the protein perforin, which integrates into the target cell membrane and forms transmembrane pores. Special proteases—granzymes—penetrate inside through these openings. Once in the cytoplasm, they destroy intracellular proteins, leading to the activation of caspase enzymes and the irreversible initiation of apoptosis.
• Osmotic lysis. The pores formed by perforin disrupt the barrier function of the membrane. Water and ions rush into the target cell in an avalanche-like manner through these pores. This causes severe osmotic shock, which ends in cell necrosis.
• Fas-dependent mechanism. A specific protein—Fas ligand—is expressed on the surface of the T-killer membrane. In turn, the corresponding Fas receptor (also known as CD95) is located on the surface of the attacked target cell. Direct interaction in the ligand-receptor system transmits a powerful signal inside the target, initiating the process of apoptosis.

The outcome of T-killer action is always the death of the target cell, representing a complex combination of necrosis and apoptosis mechanisms.

Antibody-dependent reactions and hormonal targets

In addition to direct contact with T-lymphocytes, the destruction of target cells can occur via complement-independent (antibody-dependent) reactions.

In this case, specific antibodies bind to the surface of the target cell. Killer cells (K cells) have an Fc receptor on their membrane. Physical contact occurs: the killer's Fc receptor binds to the Fc fragment of the antibody already attached to the target. In response to this interaction, the K cell secretes cytolytic substances. The result is antibody-dependent damage and complete destruction of the target cell.

In the endocrine system, interaction with targets has a different nature. A classic example is mineralocorticoids.

The molecular mechanism of their action consists in binding to specific mineralocorticoid receptors. Unlike many other hormones, these receptors are localized not on the surface, but deep in the cytoplasm of target cells.

The sites of action of mineralocorticoids are divided into two groups:

• Kidneys (main target): hormones act on the terminal part of the distal tubules and the cortical part of the collecting ducts.
• Extrerenal targets: these include sweat and salivary glands, as well as cells of the gastrointestinal tract (GIT) mucosa.

The main pharmacodynamic effect of stimulating these intracellular receptors is a significant increase in the reabsorption of sodium ions (Na^+).

Class and exam questions

What else to study on this topic

• Molecular cascades of caspase activation upon granzyme action
• The role of perforin in membrane pore formation
• Mechanisms of osmotic shock and target cell necrosis
• Secretion of cytolytic substances by K cells during antibody-dependent reactions
• Intracellular signaling pathways from mineralocorticoid receptors

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.