The intermediate filaments are an essential component of the cytoskeleton. Presently six types of intermediate filament have been identified. Type I and II are acidic and basic keratin proteins. Type III is of mesodermal origin and comprises four proteins: vimentin, desmin, glial fibrillary acidic protein (GFAP), and peripherin. Vimentin is commonly found in mesenchymal cells, desmin in muscle cells, GFAP in astrocytes, while peripherin is found in peripheral nervous system neurons (PNS). Type IV consists of three neurofilaments; NF-Light, NF-Medium, NF-Heavy, and α-internexin, nestin, and synemin. Type V consists of nuclear lamins, and Type VI comprises newly discovered phakinin and filensin protein found in the lens cells and the neuronal stem cells.
Based on their location, these filaments are broadly classified into cytoplasmic and nuclear intermediate filaments. Type I, II, IV, and VI are exclusively cytoplasmic filaments. The cytoplasmic intermediate filaments are tissue-specific. Type III filaments are found in both cytoplasm and the nucleus, while Type V is restricted to the nucleus. Several diseases are related to gene mutation encoding the different types of intermediate filaments. Some important diseases related to intermediate filaments include epidermolysis simplex bullosa, pachyonychia congenita, Hutchinson-Gilford progeria, Charcot Marie Tooth disease, and acquired partial lipodystrophy.
