Intermediate Filaments & Cell Junctions PDF

The cytoskeleton consists of microtubules, microfilaments...

The cytoskeleton consists of microtubules, microfilaments (actin filaments), and intermediate filaments (IFs) **Overview** Maintenance of cell morphology. IFs are 10 nm in diameter, stable yet dynamic Functions (not static), and non-polarized Located around the nucleus (perinuclear) and extending to the plasma membrane. Animal tissues are composed of cells organized into IFs are polymers of fibrillar proteins Animal cells in tissues: tissues and connected by the extracellular matrix (ECM) A central α-helical domain. Connective Tissue: ECM-rich, with dispersed cells. Tissue Types: **Structure and Organization** Over 50 fibrillar proteins exist, all with: Epithelial Tissue: Dense cellular sheets, minimal ECM. **Introduction** N-terminal head and C-terminal tail domains, which vary between proteins Mechanical cohesion. Six major subtypes of IFs, based on protein composition and cellular location Prevention of molecular leakage Roles of cell junctions: 1: Monomers with central α-helical rods form Facilitation of intercellular communication. coiled-coil dimers. Prevents paracellular diffusion. 2: Dimers assemble into tetramers in staggered, Role in epithelial barriers: antiparallel arrangements. Maintain polarity by segregating membrane domains **Tight Junctions** 3: Tetramers align end-to-end into **Assembly of Intermediate Filaments** protofilaments. Claudins (assembly and structure). Sealing proteins: 4: Eight protofilaments wind together in a rope- Occludins (permeability). like structure to form IFs. Connect actin filaments via cadherins. **Filament**: Eight protofilaments form a ropelike structure. Cadherins, catenins, vinculin. Components: Type I-II (Keratins): Found in epithelial cells (skin, Function in epithelial tube formation. **Adherens Junctions** Cytoskeleton – Intermediate hair, nails) Filaments Role in tissue development: Actin contraction helps epithelial sheets form tubes (e.g., neural Actin filaments form an adhesion belt, Intermediate Vimentin (Fibroblasts, leukocytes, endothelial cells). connected to cadherins and catenins tube formation) Filaments & Cell Type III: Desmin (Muscle cells). Provide strong adhesion between adjacent cells under mechanical stress Junctions ** IF Subtypes and Cell Specificity** Cell Junctions GFAP (Astrocytes, glial cells). Link intermediate filaments via cadherins (desmoglein, desmocollin) Type IV: Neurofilaments (Neurons). **Desmosomes** **Types of Cell Junctions** Cadherins (Desmoglein, Desmocollin). **Anchoring Junctions** Type V: Lamins (found in Nuclear envelope). A cytoplasmic plaque (with plakoglobin and Type VI: Nestin (found in Neural stem cells). Composed of: desmoplakin) Impart mechanical stability. Intermediate filaments (Keratin, Desmin). **Functions** Anchor desmosomes and hemidesmosomes. Link intermediate filaments to ECM via integrins **Hemidesmosomes** Caused by mutations in keratin gene, keratin Key proteins: α6β4 integrins, plectin. disrupts basal layer integrity. **Pathology** Epidermolysis Bullosa: Link actin filaments to ECM via integrins Defective keratin leads to rupturing of the basal epidermal **Focal Adhesions** layer, resulting in skin blistering under mechanical stress Key proteins: Integrins, Talin, Vinculin, α-actinin. Provides mechanical support to the nuclear Electrical coupling (e.g., cardiac muscle). envelope Enable ion and small molecule exchange, **Nuclear Lamina** Function: critical for Signal synchronization in tissues like the heart Phosphorylation causes lamina fragmentation, leading and smooth muscle to nuclear envelope disassembly during mitosis **Gap Junctions** Made of connexons, each formed by 6 Cellular integrity. Structure: SUN and KASH proteins bridge the nucleus and connexins **Linker Proteins** Mechanical stress adaptation.  cytoplasm, ensuring: Nuclear-cytoplasmic coordination Cancer metastasis (e.g., loss of E-cadherin in adherens junctions). Loss of adhesion molecules can lead to: **Pathologies** Cardiomyopathies due to disrupted junctions. 