Intermediate Filaments 

Intermediate filaments are important components of the cell's cytoskeletal system. They may stabilize organelles, like the nucleus, or they may be involved in specialized junctions. They are distinguished from "thin filaments" by their size (8-10 nm) and the fact that thin filaments are obviously motile. However recent evidence indicates that Intermediate Filaments may also have dynamic properties. Read pages 44-46 in Gartner and Hiatt

  This figure shows the rope-like characteristics of IFs.  Note that there is some organizational pattern.

You can also see organization in the following cartoon. By the end of the unit, you will be able to identify the regions in this pattern:

Test yourself:  How much do you know about Intermediate filaments? 

In the cartoon below, note the concentration of filaments at cell junctions or at the base. Note the filaments around the nucleus.  What is the significance of these sites?

1. Intermediate filaments are one of three components of the cytoskeletal system. Name all three.
2. What are basic characteristics of Intermediate Filaments?
3.  A number of different forms of keratins are found?  How is this useful for tumor diagnoses? 
4. Where are lamins found?  How are they unique? What might happen if you added antibodies to lamins during metaphase or prophase?
5. What types of junctions are formed with keratin or desmin as the intermediate filaments? Describe the components and structure of each junction. 
6. Patients who make antibodies to cadherins found in epithelial cells of skin have defects in what type of junction?
7. Types III and IV Intermediate filaments are found in what types of tissue? 

Intermediate filaments are one of three types of cytoskeletal elements.  The other two are thin filaments (actin) and microtubules.  Frequently the three components work together to enhance both structural integrity, cell shape, and cell and organelle motility.  Intermediate filaments are stable, durable. They range in diameter from 8-10 nm (intermediate in size compared with thin filaments and microtubules). They are prominent in cells that withstand mechanical stress and are the most insoluble part of the cell. The intermediate filaments can be dissociated by urea.   

Intermediate Filament polymerization

The monomer:

Each intermediate filament monomer consists of an alpha helical rod domain which connects the amino (head) and carboxyl (tail) terminals.  The figure below (16-12 from Alberts et al, Molecular Biology of the Cell, Garland Publishing, NY, 1996) shows some examples of monomers.

Formation of the protofilament.

The rods coil around another filament like a rope to form a dimer. The N and C terminals of each filament are aligned. Some Intermediate filaments form homodimers; other form heterodimers.  

These dimers then form staggered tetramers that line up head-tail. Note that the carboxy and amino terminals project from this protofilament. This tetramer is considered the basic subunit of the intermediate filament.   The final 10 nm filament is a helical array of these tetramers.

Regulation of assembly or disassembly of intermediate filaments:    Most of the Intermediate filaments are fully polymerized.  However, there is evidence that even these stable structures have dynamic properties. There is some free tetramer in the cytoplasm as if this is the basic subunit for assembly of new filaments.  Also, if one phosphorylates serine residues in the amino terminals one can cause disassembly.

Types of Intermediate filaments

Lamins (Type V)
In evolution, this was probably the first intermediate filaments made. They have a very long rod domain and carry a nuclear transport signal because they reside in the nucleus just under the nuclear envelope.  They are continuous except for a break at the sites of the nuclear pore complex.  They form a lattice-like array , just inside the nuclear envelope.  They are difficult to distinguish from the nearby dense heterochromatin.    Lamins are vital to the re-formation of the nuclear envelope after cell division.

Lamins are phosphorylated at the end of prophase and this causes them to disassemble as the nuclear envelope also breaks down.  Then, the phosphate is removed just before the nuclei of the daughter cell forms and the lamin filaments reassemble around each set of chomosomes, under each daughter cell's inner nuclear membrane.  One can block this process by adding antibodies to lamins before the nuclear membranes are formed.

Keratins (Type I and II)
Types I and II intermediate filaments are acidic and basic keratins, respectively.  Their monomers are found in the same cells and the dimers must contain one of each type (a heterodimer).  Keratins also have subtypes that are unique to different epithelial cells (bladder, skin, etc) or even different subsets of one cell type (like basal epidermal cells). This is useful in detection of the origin of cells in a tumor, especially cells that have metastasized. 

Specialized junctions
In epithelia, keratins intermediate filaments form junctions that hold cells together (desmosomes), or attach cells to matrix (hemidesmosomes). In muscle cells, the intermediate filaments that form the desmosome are "desmins".

Desmosomes: Two plaques on adjacent cells (containing desmoplakin and other proteins) are connected by cadherin molecules.  These molecules are linked by calcium.  The intermediate filaments loop into the plaques spreading out into the cytoplasm.  This links two cells together structurally.         

These cells in the above photomicrograph are from skin. They look as if they have projected spines that touch spines from adjacent cells.  Actually these are sites of desmosome connection which is a vital junction in the skin.  The fixation technique has caused the cells to shrink, leaving the  connection sites visible.  An electron micrograph showing a desmosome is seen below. The intermediate filaments are looping in in almost a parallel fashion.

Patients who make antibodies to cadherin molecules will have weak or absent desmosomes and the skin will form blisters.  These fluid filled areas will lie in the regions where the cells with spines are found (see previous photo).

Hemidesmosomes: Are sites of connections at the base of an epithelial cell with the matrix. The cartoon below shows the components.  Intermediate filaments are stuck in a plaque (like the desmosome plaque) and Integrin molecules (receptors for matrix proteins) help connect the site with the matrix. The cartoon below shows the structure of a hemidesmosome.

Type III Intermediate Filaments
Found in a variety of cell types. Each is unique to that cell type and used to identify tissue containing that cell type.
Vimentin is found in cells derived from mesoderm: fibroblasts, endothelial cells, white blood cells. Desmin is found in muscle cells, connecting Z disks and may connect center of contractile units.  It is also found connecting to desmosomes in specialized junctions (cardiac muscle). Glial fibrillary acidic protein is found in glial cells in the central nervous system.

Type IV Intermediate filaments
Include Neurofilament L, M, or H (named for low, medium or high molecular weight.  These neurofilaments are linked by plectin cross bridges to each other and to microtubules. This adds to strength and spacing. Neurofilament proteins add to the diameter of the axon and therefore influence its function (larger axons conduct faster).    Another type IV is "internexin" and some nonstandard IV's are found in lens fibers of the eye (filensin and phakinin).