Premed · Premed · Cell Biology
Lecture 18: The Cytoskeleton: Intermediate Filaments
Cell Biology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the general structure and assembly of intermediate filaments
- Compare and contrast the six major classes of intermediate filament proteins
- Explain how intermediate filaments provide mechanical strength to cells and tissues
- Discuss diseases caused by intermediate filament mutations
- Explain the regulation and dynamic properties of intermediate filaments
Lecture Content
I. General Structure and Properties
Intermediate filaments (IFs) are approximately 10 nm in diameter, placing them between actin filaments (7 nm) and microtubules (25 nm) in size. Their primary function is to provide mechanical strength and structural integrity. IFs are the most durable of the three cytoskeletal systems, able to resist tensile (stretching) forces while remaining flexible.
Several key differences distinguish IFs from actin filaments and microtubules. IFs have no polarity -- they lack distinct plus and minus ends. Their assembly does not require nucleotide binding or hydrolysis (no ATP or GTP is needed). No motor proteins walk along IFs, unlike actin (which has myosins) and microtubules (which have kinesins and dyneins). IFs are far more chemically heterogeneous, encoded by approximately 70 different genes in humans. They turn over more slowly than actin or microtubules, though they are not static. Importantly, different IF proteins are expressed in different cell types, providing tissue-specific expression that serves as useful diagnostic markers, particularly in cancer pathology.
II. Intermediate Filament Assembly
All IF proteins share a common structural motif consisting of a central alpha-helical rod domain of approximately 310 amino acids (about 45 nm long), containing heptad repeats that form a coiled-coil. This rod domain is highly conserved in length across all IF types. Flanking the rod domain are globular head (N-terminal) and tail (C-terminal) domains that vary in size and sequence and determine the specific functions and interactions of each IF type.
Assembly proceeds through a well-defined hierarchical pathway. First, two IF polypeptides wrap around each other in parallel (same direction) to form a coiled-coil dimer. Two dimers then associate in an anti-parallel, staggered arrangement to form a tetramer. This anti-parallel arrangement is what creates the non-polar character of the filament. Approximately eight tetramers associate laterally to form a unit-length filament (ULF). Finally, ULFs anneal end-to-end and undergo radial compaction to produce the mature approximately 10 nm filament. This entire assembly process is spontaneous at physiological ionic strength and requires no nucleotide.
<image>Intermediate filament assembly pathway. Panel A: Domain structure of an IF monomer — N-terminal head domain (globular), central alpha-helical rod domain with coiled-coil heptad repeats (four segments 1A, 1B, 2A, 2B separated by linkers), and C-terminal tail domain (globular). Panel B: Step-by-step assembly — two monomers form a parallel coiled-coil dimer; two dimers associate anti-parallel and staggered to form a tetramer (non-polar); eight tetramers associate laterally to form a unit-length filament (ULF); ULFs anneal end-to-end and compact to form the mature ~10 nm filament. Panel C: Electron micrograph-style depiction of the final ropelike filament emphasizing the non-polar, flexible structure.</image>
III. Classes of Intermediate Filament Proteins
IF proteins are grouped into six major types based on sequence homology. Type I and II keratins constitute the largest IF family, with approximately 54 genes in humans. Type I keratins are acidic (K9-K28, K31-K40) and type II are basic or neutral (K1-K8, K71-K86). They always form obligate heterodimers of one type I and one type II keratin. Epithelial keratins include K8/K18 in simple epithelia, K5/K14 in the basal layer of stratified epithelia, and K1/K10 in suprabasal differentiated layers. Hair keratins are hard keratins extensively cross-linked by disulfide bonds.
Type III IF proteins include vimentin, desmin, GFAP, and peripherin. Vimentin is expressed in mesenchymal cells (fibroblasts, endothelial cells, leukocytes) and is the most widely distributed IF protein. It serves as a marker for mesenchymal-origin tumors (sarcomas) and is upregulated during epithelial-to-mesenchymal transition (EMT). Desmin is found in muscle cells, where it connects sarcomeres at Z-discs and links to desmosomes. GFAP (glial fibrillary acidic protein) is specific to astrocytes and glial cells and serves as a diagnostic marker for gliomas. Peripherin is expressed in peripheral neurons. Unlike keratins, type III proteins can form homodimers.
Type IV includes neurofilaments (NF-L, NF-M, NF-H) and alpha-internexin, found in neurons and abundant in axons. NF-H and NF-M have long C-terminal tail domains with phosphorylation sites. Neurofilaments determine axon caliber (diameter), which in turn affects nerve conduction velocity. Nestin (type VI) is expressed in neural stem and progenitor cells and serves as a stem cell marker.
Type V comprises the nuclear lamins (Lamin A, B1, B2, and C; Lamin A and C are splice variants of the LMNA gene). Lamins form the nuclear lamina, a meshwork lining the inner nuclear membrane that provides structural support, anchors chromatin, and organizes nuclear pore complexes. The lamina is disassembled during mitosis by CDK1-mediated phosphorylation (enabling nuclear envelope breakdown) and reassembled afterward by phosphatase-mediated dephosphorylation.
IV. Intermediate Filaments and Mechanical Integrity
IFs function as a distributed network that absorbs and distributes mechanical stress throughout cells and tissues. They achieve this through connections to other structures. Desmosomes are cell-cell junctions that anchor the IF networks of adjacent cells, with desmoplakin linking the IF network to the desmosomal cadherins (desmoglein and desmocollin). This creates a continuous IF network spanning entire tissues. Hemidesmosomes are cell-ECM junctions that anchor IFs to the basement membrane, with plectin and BP230 linking keratins to integrin alpha6beta4.
Plectin is a giant cross-linking protein (approximately 500 kDa) that connects IFs to actin filaments, microtubules, and adhesion complexes, serving as a critical integrator of the three cytoskeletal systems. IFs exhibit viscoelastic behavior, stiffening under strain (a property called strain-hardening) that protects cells from large deformations.
<image>Intermediate filament networks in epithelial tissue. Panel A: Two adjacent epithelial cells connected by desmosomes — keratin IF networks (green) span each cell and are anchored at desmosomal junctions (desmoplakin linking keratins to desmoglein/desmocollin transmembrane cadherins). At the basal surface, hemidesmosomes anchor keratins to the basement membrane via plectin/BP230 and integrin alpha6beta4. Panel B: The role of plectin as a cytoskeletal cross-linker — plectin (red) connecting intermediate filaments (green), actin filaments (orange), and microtubules (blue) into an integrated network. Panel C: Diagram showing the tissue-wide IF network created by desmosome-linked keratin filaments across multiple cells, illustrating how mechanical force is distributed across the entire tissue rather than concentrated in a single cell.</image>
V. Intermediate Filament Diseases
Mutations in IF genes cause diseases of mechanical fragility that dramatically illustrate the structural role of these filaments. Epidermolysis bullosa simplex (EBS) results from mutations in K5 or K14, the keratins of basal keratinocytes in stratified epithelia. The mutant keratin network cannot withstand even mild mechanical stress, causing basal cells to rupture and producing skin blistering. The disease is autosomal dominant due to the dominant-negative effect of mutant subunits on filament assembly. Severity correlates with the position of the mutation in the rod domain, with mutations at the most conserved helix boundary motifs causing the most severe disease.
Alexander disease is caused by mutations in GFAP, the astrocyte IF protein. Mutant GFAP forms aggregates called Rosenthal fibers in astrocytes, leading to progressive leukodystrophy and macrocephaly in the infantile form. Desmin myopathies, caused by mutations in desmin, disrupt sarcomere organization and produce skeletal and cardiac myopathy.
Laminopathies are a remarkable group of diseases caused by mutations in the LMNA gene. These include Emery-Dreifuss muscular dystrophy (skeletal and cardiac muscle degeneration), dilated cardiomyopathy (heart failure), familial partial lipodystrophy of the Dunnigan type (loss of subcutaneous fat), and Hutchinson-Gilford progeria, a premature aging syndrome caused by a point mutation that creates an aberrant splice site, producing a truncated lamin A protein called progerin. Progerin retains a farnesyl group that keeps it permanently anchored to the nuclear membrane, producing misshapen nuclei, defective DNA repair, and dramatically accelerated aging. It is remarkable that mutations in a single gene can cause such diverse diseases, reflecting the tissue-specific functions of lamin A/C.
<image>Intermediate filament diseases. Panel A: Epidermolysis bullosa simplex — normal skin (left) with intact keratin IF network in basal keratinocytes versus EBS skin (right) with collapsed/aggregated keratin filaments and cell rupture at the basal layer, forming a blister cavity. Mutation site in K14 rod domain indicated. Panel B: Laminopathies — normal nucleus (left) with smooth nuclear lamina and even lamin A/C staining versus progeria nucleus (right) with lobulated, misshapen nuclear envelope and aggregated progerin (truncated lamin A with farnesyl group). Panel C: Table-style summary of IF diseases: tissue, IF protein mutated, disease name, and key phenotype for EBS (K5/K14), Alexander disease (GFAP), desminopathy (desmin), and progeria (lamin A).</image>
VI. Regulation and Dynamics of Intermediate Filaments
Although IFs are more stable than actin or microtubules, they are not static structures. Phosphorylation is the primary regulatory mechanism. Phosphorylation of serine and threonine residues in the head domain by kinases including PKC, PKA, CaMKII, and CDK1 promotes disassembly. CDK1-mediated phosphorylation of nuclear lamins during mitosis drives nuclear lamina disassembly and nuclear envelope breakdown. Vimentin phosphorylation during mitosis ensures proper redistribution to daughter cells.
IF dynamics in vivo, as revealed by FRAP studies, show that subunit exchange does occur, with a pool of non-filamentous IF precursors (soluble tetramers and ULFs) maintained in the cytoplasm. IFs can be transported along microtubules by kinesin and dynein motors, which move IF precursors to where they are needed. Additional modifications such as sumoylation of lamins influence nuclear organization. IFs also respond to cellular signals, as exemplified by vimentin reorganization during wound healing and cell migration.
