Target Discovery and Validation Reviews and Protocols

Keratin Transgenics and Knockouts 209
or constitutively K1, K2e, K9, and K10 in mouse and human (39–41). More
related to epidermal appendages, the novel keratin 17 (K17n) gene is mainly
expressed in nail tissue as well as in fili- and fungiform papillae of the oral
mucosa (36).
Nevertheless, the aforementioned concept of “keratin expression pairs” is
proven by the exception that there is no mouse ortholog to the human K3 gene
(3). In the differentiated layers of the human cornea, both K3 and K12 are abundantly
expressed; the former keratin has to be replaced by K5 in the mouse (42).
During embryonic development, keratin protein expression starts very early
with K8 and K18 at ED2.5 (32) being detectable in the surface ectoderm
(43,44) and is followed by K5 (ED9.25) and K14 (ED9.75) (32). Shortly afterward,
at ED11.5, keratins K6a, K6b, K16, and K17 appear, even before the
onset of stratification at approx ED13.5 in the mouse (45). In this respect, it is
intriguing that the mouse K14 gene might be a direct target of the p53 tumor
suppressor homolog p63 because K14 was expressed at very low levels in the
single layer of p63-deficient epidermis of mouse (46).
1.1.4. Keratin Function in Cell Signaling and Keratinopathies
Mutations within keratin genes lead to various epidermal diseases. The
diseases have different pathologies mainly caused by cell fragility, which
could be accompanied by a disturbance of the normal cell signaling (1,47,48).
An electronic database including various keratin mutations is available: at
www.interfil.org.
The most prominent K14R125C mutation not only leads to EBS (Fig. 3A)
because of keratin (IF) aggregate formation coinciding with decreased resilience
to mechanical forces but also to a weakening of the K14–tumor necrosis factor
(TNF)α receptor-associated death domain (TRADD). This weakening leaves
transfected keratinocytes more susceptible for caspase-8–mediated apoptosis
caused by an alleviated cytoprotection mediated by K14–TRADD interactions
(49). Furthermore, cell culture studies with these transfected keratinocytes
revealed that keratin aggregates are in a dynamic equilibrium with soluble subunits,
suggesting that dominant negative mutations act by altering cytoskeletal
dynamics and solubility largely depending on the “mutant to wild-type” ratio
(19). Keratin mutations render keratinocytes more susceptible to osmotic shock
and activate the stress-activated protein kinase/c-Jun NH 2 -terminal kinase (JNK)
pathway (50).
Generally, point mutations in or truncation of epidermal keratins result in visible
monogenic keratinopathies, whereas phenotypes of mutated embryonic and
internal epithelial keratins are much more subtle. Depending on the population,
K8 as well as K18 gene mutations (see Tables 1 and 2) have been associated
with cryptogenic cirrhosis, liver disorders, and chronic pancreatitis as well as