micro part 2 immunology
Immunology
Cell surface molecules and receptors
i. Definition, general Structure and mechanism (dimerization and rotation), components of signal transduction (extracellular signaling molecule, receptor proteins, intracellular signaling proteins and target proteins)
Signal Transduction in Microbiology
Definition:
- Signal transduction is the process by which cells receive and respond to external signals or stimuli. It involves the conversion of extracellular signals into intracellular responses.
General Structure and Mechanism:
Dimerization and Rotation: In many signal transduction pathways, receptor proteins dimerize (form pairs) upon binding to an extracellular signaling molecule. This dimerization is often critical for initiating the signaling cascade.
- Components of Signal Transduction:
- Extracellular Signaling Molecule: This is the molecule that originates outside the cell and binds to the cell's receptor protein. Examples include hormones, growth factors, neurotransmitters, and cytokines.
- Receptor Proteins: These are typically transmembrane proteins that span the cell membrane. They can be classified into various types, such as G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs), and ion channel receptors.
- Intracellular Signaling Proteins: Once the receptor is activated by binding to the signaling molecule, it initiates a series of intracellular events. This involves various signaling proteins like kinases, phosphatases, and second messengers (e.g., cyclic AMP or calcium ions) that relay the signal to the target proteins.
- Target Proteins: These are the ultimate effectors of the signal transduction pathway. They can be enzymes, transcription factors, or other proteins that carry out specific cellular responses.
Key Concepts:
- Phosphorylation: Phosphorylation is a common post-translational modification in signal transduction. Kinases add phosphate groups to proteins, often activating or deactivating them. Phosphatases remove phosphate groups.
- Second Messengers: These are small molecules like cyclic AMP (cAMP) and calcium ions (Ca²⁺) that amplify and transmit the signal within the cell.
- Amplification: Signal transduction pathways often involve amplification steps, where one activated molecule can activate multiple downstream molecules, leading to a robust cellular response.
- Integration: Cells can integrate multiple signals simultaneously to make decisions about their behavior. Integration can lead to diverse and complex responses.
Examples:
- RTK Signaling: Receptor tyrosine kinase signaling involves growth factors binding to RTKs, leading to cell growth and division.
- GPCR Signaling: G protein-coupled receptors activate intracellular pathways through G proteins, which can regulate various cellular functions, including neurotransmission and immune responses.
Regulation:
- Signal transduction pathways are tightly regulated to ensure proper cellular responses and prevent overstimulation. This regulation includes feedback loops, desensitization, and degradation of signaling components.
Extracellular Signaling Molecule: This represents the signaling molecule originating from outside the cell.
Receptor Proteins: These are depicted on the cell surface, indicating that they are transmembrane proteins or proteins embedded in the cell membrane. Receptor proteins bind to the extracellular signaling molecule.
Intracellular Signaling Proteins: These proteins are situated within the cell and receive signals from receptor proteins. They can include kinases, phosphatases, and second messengers.
Target Proteins: These are the ultimate effectors of the signal transduction pathway. They can be enzymes, transcription factors, or other proteins that carry out specific cellular responses.
ii. Adhesion molecules in immune activation, structure and function of B Cell Receptor, TCR‐CD3 complex, Toll‐like receptors, Cytokine receptors, G‐protein coupled receptors
Adhesion Molecules in Immune Activation:
Structure and Function:
- Adhesion molecules play a crucial role in immune activation by facilitating interactions between immune cells and their targets, such as pathogens or other cells.
- Selectins: These are cell surface glycoproteins that mediate the initial weak interactions between immune cells and endothelial cells during inflammation. They include E-selectin (endothelial), P-selectin (platelet and endothelial), and L-selectin (leukocyte).
- Integrins: Integrins are transmembrane proteins that provide stronger adhesion by binding to extracellular matrix proteins (e.g., fibronectin) and cell surface molecules (e.g., ICAM-1). Examples include LFA-1 (CD11a/CD18) on leukocytes.
- ICAMs: Intercellular Adhesion Molecules (ICAMs) are adhesion receptors on endothelial cells and leukocytes. They bind to integrins to facilitate cell-cell interactions.
- VCAM-1: Vascular Cell Adhesion Molecule-1 is an endothelial adhesion molecule that interacts with integrins on leukocytes, promoting their adhesion to endothelial cells.
Structure and Function of B Cell Receptor (BCR):
Structure:
- The B Cell Receptor consists of two components:
- Membrane-Bound Immunoglobulin (Ig): This is the B cell's surface antibody and is specific to antigens.
- Igα and Igβ: These are associated with the Ig molecule and are involved in signal transduction.
Function:
- The BCR recognizes and binds to specific antigens, leading to B cell activation.
- Upon antigen binding, the BCR initiates signaling cascades that ultimately result in B cell proliferation and antibody production.
Structure and Function of TCR-CD3 Complex:
Structure:
- The T Cell Receptor (TCR) is a complex composed of:
- α and β (or γ and δ) chains: These chains contain variable regions that determine antigen specificity.
- CD3 Complex: CD3ε, CD3δ, CD3γ, and CD3ζ chains are associated with the TCR and are important for signal transduction.
Function:
- The TCR recognizes antigenic peptides presented by major histocompatibility complexes (MHC) on antigen-presenting cells (APCs).
- TCR engagement triggers intracellular signaling events via the CD3 complex, leading to T cell activation and immune responses.
Toll-Like Receptors (TLRs):
Structure and Function:
- Toll-Like Receptors (TLRs) are a class of pattern recognition receptors (PRRs) expressed on immune cells.
- TLRs recognize specific pathogen-associated molecular patterns (PAMPs) present on microbes, initiating an innate immune response.
- Activation of TLRs leads to the production of proinflammatory cytokines and type I interferons.
Cytokine Receptors:
Structure and Function:
- Cytokine receptors are cell surface receptors that bind cytokines, small signaling proteins involved in immune responses.
- They trigger various intracellular signaling pathways upon cytokine binding, regulating immune cell proliferation, differentiation, and function.
- Examples include IL-2 receptors and IL-6 receptors.
G-Protein Coupled Receptors (GPCRs):
Structure and Function:
- GPCRs are a large family of cell surface receptors involved in a wide range of immune responses, including chemotaxis and inflammation.
- They activate intracellular signaling pathways through G proteins upon ligand binding.
- Examples in immune cells include chemokine receptors, which mediate cell migration, and histamine receptors, which are involved in allergic responses.
Understanding these adhesion molecules and receptor complexes is essential for comprehending how immune cells interact with pathogens and other immune cells during an immune response
Adhesion Molecules
|
+----------|----------+
| | |
Selectins Integrins ICAMs
- Adhesion Molecules: Central category representing molecules that facilitate cell-cell interactions.
- Selectins: Subgroup of adhesion molecules involved in initial weak interactions during inflammation.
- Integrins: Subgroup of adhesion molecules that provide stronger adhesion.
- ICAMs: Intercellular Adhesion Molecules involved in cell-cell adhesion.
Structure and Function of B Cell Receptor:
- B Cell Receptor (BCR): Consists of membrane-bound immunoglobulin (specific to antigens) and Igα/Igβ (involved in signal transduction).
Structure and Function of TCR-CD3 Complex:
- T Cell Receptor (TCR): Composed of α and β (or γ and δ) chains specific to antigens.
- CD3 Complex: Associated with TCR and crucial for signal transduction.
Toll-Like Receptors (TLRs):
- Toll-Like Receptors (TLRs): Recognize pathogen-associated molecular patterns (PAMPs) and initiate intracellular signaling, leading to immune responses.
Cytokine Receptors:
- Cytokine Receptors: Cell surface receptors that bind cytokines, regulating immune cell functions.
G-Protein Coupled Receptors (GPCRs):
iii. Signal transduction pathways: IL‐2 pathway (JAK/STAT, Ras/MAP Kinase Pathways, TCR-CD3 activation pathway)
IL-2 Pathway (JAK/STAT Pathway):
Overview:
- IL-2 (Interleukin-2) is a cytokine produced by activated T cells.
- The IL-2 pathway is crucial for the regulation of immune responses and T cell proliferation.
Signal Transduction:
IL-2 Binding: IL-2 binds to its receptor (IL-2R) on the T cell surface.
Receptor Activation: This binding induces conformational changes in the receptor, leading to the activation of Janus kinases (JAKs) associated with the receptor.
JAK Activation: Activated JAKs phosphorylate the receptor, creating docking sites for Signal Transducers and Activators of Transcription (STATs).
STAT Activation: STATs are phosphorylated by JAKs and then dimerize.
Nuclear Translocation: The STAT dimers translocate to the nucleus and bind to specific promoter regions of target genes.
Gene Expression: This binding leads to the transcription of genes involved in T cell proliferation and immune regulation.
Ras/MAP Kinase Pathway:
Overview:
- The Ras/MAP Kinase pathway is a key signaling pathway involved in cell growth, differentiation, and survival.
Signal Transduction:
Receptor Activation: Various receptors, including receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs), activate the pathway.
Ras Activation: Activated receptors lead to the activation of the small GTPase protein Ras.
MAP Kinase Cascade: Ras activates a series of kinases, including Raf, MEK (MAP kinase kinase), and ERK (extracellular signal-regulated kinase), in a linear cascade.
Phosphorylation: ERK phosphorylates downstream target proteins, including transcription factors.
Nuclear Translocation: Phosphorylated transcription factors enter the nucleus and regulate gene expression, impacting cell growth and differentiation.
TCR-CD3 Activation Pathway:
Overview:
- The TCR-CD3 activation pathway is crucial for T cell activation and immune responses.
Signal Transduction:
Antigen Recognition: The TCR recognizes antigenic peptides presented by MHC molecules on antigen-presenting cells (APCs).
CD3 Activation: TCR engagement triggers the activation of the CD3 complex associated with the TCR.
Intracellular Signaling: The CD3 complex contains ITAMs (immunoreceptor tyrosine-based activation motifs) that are phosphorylated by Lck kinase, a cytoplasmic kinase associated with the TCR complex.
ZAP-70 Activation: Phosphorylated ITAMs recruit and activate ZAP-70 kinase, which initiates downstream signaling events.
Activation of Signaling Pathways: ZAP-70 activates multiple signaling pathways, including the Ras/MAP Kinase pathway and the Calcineurin-NFAT pathway, leading to gene expression and T cell activation.
IL-2 Production: Activated T cells may produce IL-2, which can further activate immune responses.
These signal transduction pathways are central to immune responses and cellular regulation. Understanding the molecular events within these pathways is essential for grasping how immune cells respond to antigens and initiate immune reactions.
IL-2 Pathway (JAK/STAT Pathway):
- IL-2: Interleukin-2, the cytokine produced by activated T cells.
- IL-2R: IL-2 receptor on the T cell surface.
- JAK: Janus kinases activated upon IL-2 binding.
- STAT: Signal Transducers and Activators of Transcription proteins, phosphorylated by JAK.
- Nucleus: Represents the nucleus where STAT dimers translocate to and regulate gene expression.
Ras/MAP Kinase Pathway:
- Receptor: Represents various receptors like receptor tyrosine kinases (RTKs) or G protein-coupled receptors (GPCRs) that activate the pathway.
- Ras: Small GTPase protein activated by receptors.
- Raf, MEK, ERK: Kinases in a linear cascade.
- Nucleus: Where phosphorylated transcription factors regulate gene expression.
TCR-CD3 Activation Pathway:
- Antigen: Represents antigen recognition.
- TCR: T Cell Receptor recognizing antigenic peptides.
- CD3 Complex: Involved in TCR activation.
- Lck: Cytoplasmic kinase associated with TCR.
- ZAP-70: Kinase activated by Lck, initiating signaling pathways.
- Signaling Pathways: Represents various downstream pathways.
- Nucleus: Where gene expression is regulated.
Comments
Post a Comment