Target Discovery and Validation Reviews and Protocols

216 Vijayaraj, Söhl, and Magin
of the TNF- or Fas-mediated apoptosis caused by a putative loss of interaction
with the K8-mediated IF network might lead to an atrophy and degradation of
the placenta (59,61). This effect possibly could be proven after rescuing of the
embryonal lethality in a TNFα-deficient mice strain.
1.1.5.2. BASAL STRATIFIED EPITHELIA
In basal keratinocytes, three keratins (K5, K14, and K15) are coexpressed
(8). They are linked to hemidesmosomes via plectin and bullous pemphigoid
antigen-1 (66,67) and to desmosomes via desmoplakin (68) and plakophilin
(69). To investigate the function of these keratins in the basal epidermis, mouse
models have been established by gene targeting: K14-deficient mice, K14(–/–),
of a mixed 129Sv/C57BL/6 background generally die within 48 h after birth
(although there are some exceptions surviving up to 3 mo). Abundance of K15,
however, is normally compensating for K14, but it is too low in neonatal mice
(35). An age- or tissue-dependent upregulation of K15 in a few K14(–/–) mice
may permit their survival up to 3 mo (35).
Subsequently, deletion of the partner keratin K5, being the only type II keratin
protein in most basal keratinocytes, should abolish the formation of Ifs, which
was done in a mixed 129Ola/BALB/c strain. Indeed, all mice die immediately
after birth (Fig. 3F) containing an extensive cytolysis in almost all basal
keratinocytes (21). Residual type I keratins K14, K15, and K17 (now without
partner keratin) aggregated along hemidesmosomes in K5(–/–) mice. Because
K5(–/–) pups are more severely affected than K14(–/–) mice, it is likely that
K5 null mutations may be lethal in human EBS patients (Fig. 3F). In contrast
to K14-deficient mice, K5(–/–)-mice showed a strong induction of the woundhealing
keratins K6 and K10 in the suprabasal epidermis of the cytolyzed compartments
(21). Very early during embryogenesis of K5(–/–)-mice, K8 is able to
compensate for K5 until its expression is downregulated at ED14.5 (32). Thus,
an embryonic keratin (K8) is at least partially able to compensate for an epidermal
keratin, provided that it is expressed in the same spatiotemporal manner.
Confirmation for this hypothesis can be drawn from transgenic rescue experiments,
in which the hK18 transgene was expressed in the basal epidermis of
K14(–/–) mice. Although filaments between K5 and hK18 have been generated,
they were unable to resist against mechanical stress (70).
Another, more sophisticated transgenic experiment imitated the inducible
expression of the most severe Dowling–Meara-related K14 mutation of EBS
[(R125C) in mouse (R131C) (71), 1995, OMIM 131760] in mice derived from
a mixed 129Sv/C57BL/6 strain background. Heterozygous mice seemed normal
with no gross skin phenotype and no microscopic blisters. Homozygous mice,
however, were normal at birth but developed extensive blisters on the front legs,
paws, and chest and died within 2 d (72). An inducible transgenic mouse model