Histochem Cell Biol (2008) 129:735–749 Table 2 Examples of IF-asso- ciated proteins Type of interaction Examples Anchoring Cytolinker Chaperones Kinases Adaptor proteins Membrane proteins Apoptotic proteins Motor proteins Desmoplakin, BPAG1, -dystobrevin Plectin, Filaggrin, Hsp27, -B-crystallin, Hsp70 PKC, Cdk5 14-3-3 protein, AP-3 Polycystin-1 TRADD, TNFR2, C-Flip, Caspase3/9 Dynein, Kinesin 737 Function Tissue architecture Cellular architecture Protein folding IF regulation, phosphate sink, cell cycle Multiple eVects Unknown Apoptosis regulation Movement of IF components phosphorylation through associated kinases does not only regulate IF properties, but IFs also serve as phosphate “sponge” thereby preventing activation of other, potentially pro-apoptotic, substrates (Omary et al. 2006; Ku et al. 2007). In addition to that, IFs also directly participate in apoptosis regulation through binding of several apoptosis- related molecules (Marceau et al. 2007). In contrast to the actin and tubulin system, IFs emerged later in the evolution and are important supportive elements of the cell rather than their essential components. There- fore, IF variants are observed in various human diseases, which reXect their tissue speciWc distribution, whereas only few actin and tubulin variants have been described so far, likely due to their embryolethality (Ku et al. 1999; Omary et al. 2004). Currently, more than 30 diseases are caused by/associated with IF mutations (Omary et al. 2004). Among them, keratin-related- and lamin-related disorders are the best studied ones. For example, mutations in kera- tins 5/14 cause a blistering skin disease termed Epider- molysis bullosa simplex, whereas mutations in lamins A and C result, among others, in diVerent diseases including premature aging, cardiomyopathy, and lipodystrophy (Omary et al. 2004; Dechat et al. 2008). The disease relevance of IFs is also highlighted by a vari- ety of IF-containing inclusion bodies, which represent the pathological hallmarks of several neurodegenerative, muscu- lar, and other disorders (Goebel 1998; Ross and Poirier 2004; Omary et al. 2004; Cairns et al. 2004). These aggregates share a variety of features such as presence of misfolded, ubiquitinated structural proteins together with variable amounts of chaperones and p62 (Kuusisto et al. 2001; Zat- loukal et al. 2002; Ross and Poirier 2004). Among the IF- related inclusions, Mallory-Denk bodies (MDBs) are the most common and also the best studied ones due to the avail- ability of animal MDB models (Denk et al. 1975; Yokoo et al. 1982; Jensen and Gluud 1994; Denk et al. 2000; Zat- loukal et al. 2007). Therefore, one focus of our review will be to describe MDB as a prototype of IF-related inclusion body, which should oVer useful insights into the formation of IF- related aggregates in multiple human diseases. Keratins as epithelially expressed IFs Keratins represent the largest subfamily of IFs consisting of >50 unique gene product members (Schweizer et al. 2006; Kim and Coulombe 2007; Godsel et al. 2008) which include 37 epithelial and 17 hair keratin members in humans (Table 1; Schweizer et al. 2006). Based on their pI, epithelial keratins can be subdivided in types I (acidic) and II (basic) corresponding to keratins 9–20 (K9¡K28) and keratins 1–8 plus keratins 71–80 (K1–K8; K71–K80), respectively (Table 1; Coulombe and Omary 2002; Schwe- izer et al. 2006). Keratins are found as obligatory type I and type II heteropolymers (i.e., consisting of at least one type I and one type II keratin) and a homozygous disrup- tion of a keratin results in degradation of its keratin partner at the protein level (Ku and Omary 2000; Omary et al. 2004). Similarly to IFs, keratins are expressed in a tissue- speciWc manner, with diVerent pairs being the major cellu- lar IFs in diVerent cell populations (Moll et al. 1982; Ku et al. 1999; Coulombe and Omary 2002). For example, “simple” (i.e., single layered) epithelia, as found in diges- tive organs, express K8 together with variable levels of K7, K18, K19, and K20 depending on the tissue (Moll et al. 1982; Ku et al. 1999; Coulombe and Omary 2002; Ku et al. 2007). In contrast, stratiWed epithelia, like epider- mis, express K5/K14 in the basal and K1/K10 in the supra- basal keratinocytes, respectively (Moll et al. 1982; Lane and McLean 2004; Coulombe and Omary 2002). Despite their similar molecular composition, “simple” and “strati- Wed” keratins are not interchangeable, as shown in K14- mice, whose phenotype was only partially restored by addition of K18 (Hutton et al. 1998). Furthermore, kera- tins have their preferential binding partners in vivo and the lack of such partner leads to their rapid degradation (Magin 1998). This contrasts with the in vitro situation, where IF assembly is more promiscuous (Hatzfeld and Franke 1985). K8/K18/K19 are promising serological markers based on their high abundance (approximately 0.3% of total liver protein) and intracellular localization under basal condi- 123