7.2 Intermediate filaments and cellular structure

Intermediate Filaments and Cellular Structure

Intermediate filaments (IFs) give cells the mechanical toughness they need to withstand physical stress. While actin filaments and microtubules handle movement and transport, IFs act more like internal cables, providing tensile strength and structural resilience. Understanding them is key to seeing how tissues hold together under force.

Structure of Intermediate Filaments

IFs have a diameter of about 10-12 nm, placing them between actin filaments (~6 nm) and microtubules (~25 nm). Unlike those other cytoskeletal components, IFs are made of fibrous proteins and are nonpolar, meaning they have no distinct plus or minus end.

Their strength comes from a hierarchical assembly process:

1. Monomers contain a central α-helical rod domain flanked by variable head and tail domains.
2. Two monomers wind around each other in parallel to form a coiled-coil dimer.
3. Two dimers align in an antiparallel, staggered arrangement to form a tetramer. This antiparallel arrangement is what makes the filament nonpolar.
4. Eight tetramers associate laterally to form a unit-length filament (ULF).
5. ULFs anneal end-to-end and compact longitudinally to produce the mature intermediate filament.

This rope-like architecture is what gives IFs their remarkable tensile strength. They can stretch significantly under force and snap back without breaking, much like a climbing rope. Notably, IF assembly does not require ATP or GTP, which distinguishes it from actin and tubulin polymerization.

Types of Intermediate Filaments

There are six major types of IF proteins, classified by their amino acid sequence and tissue distribution:

• Type I and II (Keratins): These always co-polymerize as obligate heteropolymers, with one acidic (Type I) and one basic (Type II) keratin forming each filament. Found in epithelial cells, they're the main structural proteins of skin, hair, and nails. Over 50 different keratin genes exist in humans, with specific pairs expressed in different epithelial layers.
• Type III: This group includes several proteins that can form homopolymers:
  • Vimentin is widely expressed in mesenchymal cells like fibroblasts and endothelial cells. It plays roles in cell migration and wound healing and is commonly used as a marker for mesenchymal-derived tumors.
  • Desmin is specific to muscle cells, where it links sarcomeres at Z-discs and connects the contractile apparatus to the plasma membrane, mitochondria, and nucleus. This ensures proper force transmission during contraction.
  • GFAP (glial fibrillary acidic protein) is expressed in astrocytes and other glial cells, where it maintains cell shape and provides structural support around neurons.
• Type IV (Neurofilaments): NF-L, NF-M, and NF-H (light, medium, and heavy) assemble together in neurons. Their side-arm projections, especially the heavily phosphorylated tail of NF-H, space filaments apart and directly regulate axon diameter. Larger axon diameter means faster nerve conduction velocity, so neurofilaments have a direct impact on signal transmission speed.
• Type V (Nuclear Lamins): Lamins A, B, and C form the nuclear lamina, a meshwork lining the inner nuclear membrane. Beyond structural support for the nuclear envelope, lamins anchor chromatin, help organize DNA replication, and regulate gene expression by influencing which genes are accessible to transcription machinery. Lamins are the only IFs found in the nucleus rather than the cytoplasm.
• Type VI (Nestin): Expressed in neural stem cells and progenitor cells during development. Nestin is downregulated as cells differentiate and is replaced by tissue-specific IFs. Because of this expression pattern, it's widely used as a stem cell marker.

Cellular Integrity from Filaments

IFs form a continuous network stretching from the nuclear lamina to the plasma membrane. They interact with both actin filaments and microtubules through cross-linking proteins like plectin, which physically bridges all three cytoskeletal systems into an integrated mechanical framework.

This network gives cells two critical properties:

• Tensile strength: IFs resist stretching forces. A single IF can be stretched to about 2.5 times its resting length before breaking, far more than actin or microtubules can tolerate.
• Elasticity: After deformation, IFs return to their original shape, preventing permanent damage from transient mechanical stress.

When IF proteins are mutated, cells lose this mechanical resilience. The clinical consequences map directly onto which IF type is affected:

• Keratin mutations (e.g., K5 or K14) cause epidermolysis bullosa simplex, where basal epidermal cells rupture under mild friction, leading to skin blistering.
• Desmin mutations cause desmin-related myopathy and can lead to dilated cardiomyopathy, as muscle cells lose their ability to transmit contractile force properly.
• Lamin A mutations cause a group of diseases called laminopathies, including Emery-Dreifuss muscular dystrophy and the premature aging syndrome progeria (Hutchinson-Gilford syndrome).

These diseases reinforce a core principle: the specific IF expressed in a tissue determines what kind of mechanical failure occurs when that IF is defective.

Filaments in Cell Adhesions

IFs are the cytoskeletal anchor for two specialized adhesion structures that hold tissues together under mechanical load:

Desmosomes (cell-to-cell junctions):

• Connect the IF networks of adjacent cells, creating a continuous mechanical link across the tissue.
• Transmembrane cadherins (desmoglein and desmocollin) span the intercellular space and bind to cadherins on the neighboring cell.
• On the cytoplasmic side, adaptor proteins desmoplakin and plakoglobin form the desmosomal plaque, which anchors IFs to the junction.
• Desmosomes are especially abundant in tissues under constant mechanical stress, such as the epidermis and cardiac muscle.

Hemidesmosomes (cell-to-matrix junctions):

• Anchor cells to the underlying basement membrane rather than to other cells.
• Transmembrane α6β4 integrins bind extracellular matrix components (primarily laminin) on the outside and connect to IFs on the inside.
• Cytoplasmic linker proteins plectin and BP230 (also called BPAG1) connect the integrin tails to the keratin IF network.

Disruption of either junction type leads to tissue fragility:

• Pemphigus vulgaris is an autoimmune disease where antibodies target desmosomal cadherins (desmoglein 3), destroying cell-cell adhesion and causing severe skin and mucosal blistering.
• Bullous pemphigoid involves autoantibodies against hemidesmosomal components (BP180 or BP230), detaching the epidermis from the basement membrane.
• Junctional epidermolysis bullosa results from genetic mutations in hemidesmosomal proteins or their integrin receptors, causing blistering at the dermal-epidermal junction.

The pattern across all these diseases is the same: without intact IF-based adhesions, tissues that experience mechanical stress cannot maintain their structural integrity.