05-Cell Surface Structures, Adhesion, and Signal Transduction

Glycocalyx and the Pericellular Matrix

Definition: The glycocalyx, also known as the pericellular matrix or cell coat, is a glycoprotein and glycolipid covering that surrounds the cell membranes of eukaryotic cells.
Physical Structure: The glycocalyx is approximately $200\,nm$ thick and consists of:
- Lipid Bilayer: The base membrane structure.
- Transmembrane Glycoproteins: Proteins embedded in the membrane with sugar residues extending outward.
- Adsorbed Glycoproteins: Proteins that remain associated with the surface.
- Glycolipids: Lipids with attached sugar chains.
- Transmembrane Proteoglycans: Such as Syndecan and CD44.
Functional Components:
- Proteins and Enzymes: Includes Cathepsin L, Heperanase, and Proheparanase (stored in granules).
- Polysaccharides: Heparan sulfate and Hyaluronan (including Hyaluronan fragments).
- Cell types involved: Leucocytes (bearing L-selectin), Monocytes, Platelets, and Endothelial cells.
- Extracellular Vesicles: Exosomes are also present within this matrix.
Biological Roles:
1. Microenvironment: Creating a specific local environment around the cell.
2. Protective Layer: Guarding the cell membrane.
3. Cell-Cell Recognition: Mediated by antigens and enzymes.
4. Communication: Facilitating signaling between cells.
5. Intercellular Adhesion: Helping cells stick to one another.
6. Immune Response: Specifically involved in antigen presentation.
7. Inflammation: Managing the recruitment of cells like macrophages in the subendothelial space.

Special Membrane Surface Structures: Markers and Receptors

Marker Molecules: These are glycoproteins used for:
- Cell Identification: Distinguishing different cell types.
- Antigen Property: Acting as targets for the immune system.
- Genetics: They are genetically determined.
- Cell-Cell Connection and Regulation.
MHC Proteins (Major Histocompatibility Complex):
- Also known as HLA (Human Leukocyte Antigen) complex.
- Main Function: Bind to antigens derived from pathogens and display them on the cell surface for recognition by the immune system.
- MHC-I:
  - Structure: Composed of 3 globular domains ( $\alpha_1$ , $\alpha_2$ , $\alpha_3$ ) plus $\beta_2$ microglobulin.
  - Distribution: Common on the surface of every cell.
  - Role: Endogenous antigen presentation (e.g., viruses and tumors).
- MHC-II:
  - Structure: Composed of 2 $\alpha$ chains and 2 $\beta$ chains.
  - Distribution: Found on the surface of T-lymphocytes, B-lymphocytes, and macrophages.
  - Role: Exogenous antigen presentation.
CD Proteins (Cluster of Differentiation):
- Markers used to identify species and individuals.
- Crucial for immune responses, cell adhesion, and tissue development.
- Specific Types:
  - CD 2, 3, 5, 7: Found on all T-cells.
  - CD 4: Found on all Th-cells (Helper T-cells).
  - CD 8: Found on all Tc-cells (Cytotoxic T-cells).

Receptors and Signal Transduction

Receptor Definition: Structures involved in chemical message recognition for cell-cell communication.
Locations: Can be on the membrane surface or within the intracellular space.
Roles: Cell-matrix interaction, signal recognition, and communication.
Function: Ligand recognition, binding, and signal transfer.
Ligand Types:
- Lipophilic Molecules: Can pass through the membrane to bind to intracellular receptors (e.g., steroids).
- Hydrophilic Molecules: Cannot pass through the membrane; bind to surface receptors and transmit info via secondary messengers.
The Process of Signal Transduction:
1. Reception: Ligand (primary messenger) reaches and binds to the receptor.
2. Transduction: The receptor-ligand binding triggers reactions that produce second messengers (intracellular signal transduction).
3. Response: Activation of cellular responses, such as changes in gene expression in the nucleus.
Types of Receptors:
- Ionotropic Receptors: Form an ion channel pore. Neurotransmitter binding causes immediate ion movement (fast response).
- Metabotropic Receptors: Slowly acting G-protein coupled receptors. They are linked to ion channels indirectly or utilize second messenger systems.

Second Messenger Systems

Adenylate Cyclase-cAMP System:
- Mechanism: Hormone (1st messenger) binds receptor $\rightarrow$ Receptor activates G protein ( $G_s$ ) $\rightarrow$ G protein activates Adenylate cyclase $\rightarrow$ Adenylate cyclase converts ATP to cyclic AMP (cAMP) $\rightarrow$ cAMP activates protein kinases (Inactive $\rightarrow$ Active) $\rightarrow$ Triggers cellular response (enzyme activation, secretion, opening ion channels).
- Hormones using cAMP: Epinephrine, ACTH, FSH, LH, Glucagon, PTH, TSH, Calcitonin.
Guanylate Cyclase-cGMP System:
- Mechanism: Nerve impulse/Nitric Oxide (NO) $\rightarrow$ Stimulation of Guanylate cyclase $\rightarrow$ Conversion of GTP to cGMP $\rightarrow$ cGMP-specific protein kinase activation $\rightarrow$ Decreased $Ca^{2+}$ and K+ channel activity $\rightarrow$ Smooth-muscle relaxation (e.g., erection).
- Regulation: PDE-5 (Phosphodiesterase-5) converts cGMP to $5'\,GMP$ . PDE-5 inhibitors block this to maintain relaxation.
Cell Membrane Phospholipid System:
- Mechanism: Peptide hormone binds receptor $\rightarrow$ G protein activates Phospholipase C $\rightarrow$ Phospholipase C breaks down $PIP_2$ (Phosphatidylinositol 4,5-bisphosphate) into DAG (Diacylglycerol) and $IP_3$ (Inositol triphosphate).
- Outcome: $IP_3$ releases $Ca^{2+}$ from the Endoplasmic Reticulum; DAG activates Protein Kinase C (PKC).
Calcium-Calmodulin System:
- Initiated by changes in membrane potential or hormone-receptor interactions that open calcium channels.
- Example: Activation of myosin light chain kinase, causing smooth muscle contraction.

Integration of Cells into Tissues

Cell Differentiation: Progenitor cells differentiate into distinct types with characteristic functions.
Tissue Formation: Cells of a given type aggregate to perform common functions (e.g., muscle contraction, nervous conduction).
Requirements for Tissue Cells:
- Connect to adjacent cells.
- Communicate with adjacent cells.
- Adhere to the Extracellular Matrix (ECM).

Cell Adhesion and CAMs

Cell-Cell Adhesion: Mediated by specialized integral membrane proteins called Cell Adhesion Molecules (CAMs). These often cluster into cell junctions.
Cell-Matrix Adhesion: Mediated by adhesion receptors in the plasma membrane binding to ECM components.
Role of Adhesion: Tissue aggregation, embryonic development (tube/layer formation), barrier function, membrane polarity, mechanical attachment, cell motility, signal transduction, and cancer progression.
CAM Characteristics:
- Integrated into the lipid bilayer.
- Specific to cell type and biological status.
- Composed of extracellular domains (for binding) and cytosol-facing domains.
- Intracellular domains bind to adapter proteins, which link CAMs to the cytoskeleton (actin or intermediate filaments).
Binding Interactions:
- Homotypic Adhesion: Between cells of the same type.
- Heterotypic Adhesion: Between cells of different types.
- Homophilic Binding: CAM binds to the same kind of CAM on an adjacent cell.
- Heterophilic Binding: CAM binds to a different class of CAM.
Adhesion Strength:
- Tight/Long-lasting: e.g., metabolic cells in the liver.
- Weak/Transient: e.g., leucocytes in the blood.
- Generation methodology: Tight adhesions involve cis-oligomerization (lateral/intracellular) and trans-interaction (intercellular), creating a "zipper-like" seal (e.g., Cadherins).

Major Families of Cell Adhesion Molecules (CAMs)

Classification by Ion Dependency:
1. $Ca^{2+}$ or $Mg^{2+}-dependent$ CAMs: Binding requires these ions.
2. $Ca^{2+}/Mg^{2+}-independent$ CAMs: Can bind without these ions.
1. Integrins:
- Structure: Heterodimeric (composed of 18 types of $\alpha$ subunits and 8 types of $\beta$ subunits). At least 24 heterodimers are known.
- Function: Mediate both cell-cell and cell-matrix adhesions.
- Ion Dependency: Ligand binding requires divalent cations ( $Ca^{2+}$ , $Mg^{2+}$ ).
- Specific Examples:
  - $\alpha_6\beta_4$ : Concentrated in hemidesmosomes to adhere epithelial cells to the basal lamina.
  - Mac1 ( $\alpha_M\beta_2$ -integrin): Complement-receptor on macrophages.
  - LFA1 ( $\alpha_L\beta_2$ -integrin): Expressed on leukocytes; facilitates attachment to endothelial surfaces for emigration from blood vessels (diapedesis).
2. Cadherins:
- Structure: Glycoproteins with transmembrane domains and repeating cadherin domains.
- Ion Dependency: Strictly $Ca^{2+}$ -dependent; $Ca^{2+}$ binding causes a conformational change in the extracellular domain allowing interaction.
- Types:
  - E-Cadherins: Epithelial cells; primary role in zonula adherens.
  - N-Cadherins: Neuronal and muscle cells.
  - P-Cadherins: Placenta cells.
3. Ig Superfamily CAMs:
- Structure: Contain the Ig domain.
- Ion Dependency: $Ca^{2+}$ -independent.
- Molecules: NCAM (Neural), ICAM (Intercellular), VCAM (Vascular), PECAM (Platelet-Endothelial).
4. Selectins:
- Structure: Transmembrane glycoproteins with a lectin-like carbohydrate-binding domain.
- Ion Dependency: $Ca^{2+}$ -dependent.
- Types:
  - E-selectin: Found on endothelial cell surfaces; responsible for inflammatory reactions.
  - L-selectin: Found on leukocytes; regulates initial recruitment/rolling.
  - P-selectin: Found on platelets and endothelial cells; involved in blood clotting and activation.

Extracellular Matrix (ECM)

Composition:
1. Amorphous Ground Substance: Gel-like material that absorbs water to form a cushion.
2. Meshwork of Fibers: Reinforces the ground substance.
Types and Origins:
- Mesenchymal cells: Surrounded by diffuse ECM.
- Epithelial cells: Rest on a dense sheet called the basement membrane.
- Secreted by: The cells living within the matrix.
Major ECM Molecules:
- Collagens: Most abundant protein in mammals (> $25\%$ ).
  - Fibrillar types (I, II, III, V, XI): Provide framework for bones, skin, and connective tissue.
  - Nonfibrillar (Type IV): Main component of the basal membrane.
- Adhesive Glycoproteins:
  - Fibronectin: Linked to motility; has binding sites for both cells and other ECM proteins.
  - Laminins: Abundant in basement membranes; promote cell adhesion.
- Glycosaminoglycans (GAGs): Long unbranched polysaccharides (amino sugar + uronic acid). Includes Hyaluronic acid, Chondroitin sulfate, Heparin, and Heparan sulfate.
- Proteoglycans: Core proteins with covalent GAG side chains. They attract $Na^{+}$ and water to form space-filling gels and regulate growth factors.

Cell Junctions

Tight Junctions (Zonula occludens):
- Location: Apical part of epithelial cells.
- Function: Membranes of adjacent cells fuse to encircle the cell.
- Role: Barrier to prevent leakage between cells (common in the GI tract) and prevent the diffusion of membrane proteins between apical and basal regions.
Adherens Junctions (Zonula adherens):
- Location: Just below tight junctions.
- Structure: A complex belt of actin and myosin filaments.
- Role: Controls cell shape and provides lateral connection.
Desmosomes (Macula adherens):
- Structure: "Spot rivets" involving a cytoplasmic plaque connected to intermediate filaments and intercellular protein filaments.
- Location: Common in tissues subjected to stretch, such as the skin.
Gap Junctions:
- Structure: Small channels between cells.
- Function: Intercellular communication; allow small ions to spread so cells (e.g., smooth muscle) can contract as a single unit.
Hemidesmosomes:
- Connect the basal surface of the cell to the basal lamina / connective tissue.

Recommended Literature

David L. Nelson and Michael M. Cox: Lehninger Principles of Biochemistry, Sixth Edition.
Lodish et al., Molecular Cell Biology, Fifth Edition.