M04.02.06 Cellular receptors

Understanding Cell Receptors: Structure, Types, and Function

Cell receptors are integral proteins located on the surface or within the cell membrane that play a crucial role in cell signaling and communication. They enable cells to detect and respond to various chemical signals, such as hormones, neurotransmitters, growth factors, and other ligands. The physiology of cell receptors involves several key aspects, including their structure, types, activation mechanisms, and downstream signaling pathways. Here’s a detailed overview of the physiology of cell receptors:

Structure of Cell Receptors

Cell receptors are typically transmembrane proteins consisting of extracellular, transmembrane, and intracellular domains. The extracellular domain binds specific ligands, while the intracellular domain interacts with intracellular signaling molecules or enzymes. Depending on their structure, cell receptors are classified into several major families, including G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), ion channel receptors, nuclear receptors, and cytokine receptors.

Types of Cell Receptors

G protein-coupled receptors (GPCRs): GPCRs are the largest and most diverse group of cell surface receptors. They have seven transmembrane helices and are coupled to intracellular G proteins. Ligand binding to GPCRs leads to conformational changes that activate the associated G proteins, triggering intracellular signaling cascades.

Receptor tyrosine kinases (RTKs): RTKs are transmembrane proteins with intrinsic tyrosine kinase activity. Ligand binding induces receptor dimerization, leading to autophosphorylation of tyrosine residues in the intracellular domain. These phosphorylated residues serve as docking sites for downstream signaling molecules, initiating various signaling pathways.

Ion channel receptors: Ion channel receptors are transmembrane proteins that function as ion channels. Upon ligand binding, these receptors undergo conformational changes, allowing the flow of specific ions across the cell membrane, influencing membrane potential and cellular excitability.

Nuclear receptors: Nuclear receptors are located within the cell nucleus and regulate gene expression in response to ligand binding. Upon activation, they act as transcription factors, modulating the expression of target genes.

Cytokine receptors: Cytokine receptors are involved in the immune response and cell-to-cell communication. They initiate intracellular signaling upon ligand binding and activate various pathways, including the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathways.

Activation Mechanisms

Cell receptors can be activated through various mechanisms:

Ligand binding: Ligands, such as hormones or neurotransmitters, bind to the extracellular domain of cell receptors, inducing conformational changes that initiate downstream signaling.

Dimerization: Some receptors require ligand-induced dimerization to become activated. Ligand binding brings two receptor subunits together, promoting autophosphorylation or conformational changes that initiate signaling.

Ligand-gated ion channels: Ligand binding to ion channel receptors directly regulates the opening or closing of ion channels, allowing ions to flow across the cell membrane.

Ligand-induced nuclear translocation: Nuclear receptors are activated by ligand binding, which causes their translocation into the nucleus, where they modulate gene expression.

Downstream Signaling Pathways

Once activated, cell receptors initiate various intracellular signaling pathways, leading to specific cellular responses. These pathways involve numerous intracellular signaling molecules, including second messengers (e.g., cyclic AMP, calcium ions), protein kinases, and transcription factors. The exact signaling pathways differ based on the type of receptor and ligand but often involve cascades of phosphorylation events, protein-protein interactions, and gene expression regulation.

Understanding the physiology of cell receptors is crucial for deciphering the intricate processes of cell signaling and developing targeted therapies for various diseases.